Everybody knows it's possible to convert your vintage 1953-56 Ford F-100 small window pickup cab to a big window pickup cab with relative ease. All you have to do is buy an aftermarket kit and either install it yourself, or have it installed by a capable body shop. However, when it comes to doing the same thing to a classic, Second Series '55-59 Chevrolet or GMC, it's a little more complicated, but not in the least bit impossible.
"The basic principle is pretty much the same with both the Ford F-100 and the mid-'50s GM light duty trucks," commented Cimtex Rods' Darrell Cimbanin. "Both of these cabs feature dual wall construction and are easy to work on. The only difference is that no one out there in the automotive aftermarket offers a big window conversion kit for early Chevrolet or GMC trucks."
So what's a mother to do? The good news is that there are plenty of '55-59 Chevrolet/GMC big window cabs sitting out there in junkyards (especially in the West) that, sadly, may only be good for one thing, their big back windows.
"These '55-59 Chevrolet/GMC cabs usually suffer from seriously rusted floorpans and body rot on the lower quarter panels. The good news is that when it comes to the rear window area, early Chevrolet/GMC trucks seem to exhibit little or no rust damage, and you can usually buy the rear window clip from these old cabs for approximately $250!"
However, Cimbanin cautions would-be big back window conversion specialists that in order to do the job right, it's necessary to make sure you also receive the inner wall substructure as part of your classic Chevrolet/GMC big back window clip.
"You need this piece to strengthen the cab's outer wall," said Darrell. "On small window cabs, the surrounding sheetmetal area provides the cab's rear wall with all the necessary strength it needs. However, with a big back window model, that strength is gone, so it was necessary for the factory to provide an inner wall to shore up and strengthen the back of the cab."
Installing one of these big back windows is pretty straightforward. First, you have to remove the pickup bed in order to gain full access to the back of the cab. Removing the gas tank comes next, as you're going to be working in a "hot" area. And finally, removing the small back window glass and the factory bench seat pretty much clears the way for reconstructive surgery.
These before and after photos...
These before and after photos clearly show the visual difference between '55-59 Chevrolet and GMC small and big back window cabs....
...Visibility is enhanced...
...Visibility is enhanced 200-plus percent, and it looks great. So let the sunshine in!
The work begins by removing...
The work begins by removing the bed and gas tank on Austin, Texas, fireman Mark Gusterson's small window '58 GMC half-ton.
First, our installers draw a cut line around the inside circumference of the truck cab's back window area, remembering this is double wall material. Basically, the cut line is drawn along the body and door jamb seam lines using a black felt tip marker.
With that done, the next order of business is drawing a cut line around the outside circumference of the small back window. Darrell draws his cut line just a tad above the stainless steel exterior trim mounting holes on the cab to allow for enough material to lap over.
However, just prior to actually cutting into virgin sheetmetal, the Cimbanins first reinforce the lower inside section of the cab's rear wall with three pieces of steel rod in order to maintain the rear wall's shape during the crucial cutting process. With that done, Tim Cimbanin tackles the cab's interior using a Sears Craftsman electric cutter.
Once the interior cuts have been made, it's time to tackle the outside of the cab. Cimbanin starts from the bottom trim line and works his way around the circumference of the truck cab. Once the cuts are made, the small back window panel can be easily removed.
The next order of business is trimming away the excess material from the outside of the donor clip. First, the Cimbanins draw a second set of cut lines, only this time just below the stainless steel trim mounting tab holes so the sheetmetal overlaps when the two pieces are joined.
With that done, the inner sheetmetal is again trimmed to size following the cut lines drawn and cut using the old panel. Once trimmed to satisfaction, all that remains is to fit the new panel into the opening. Obviously, some minor trimming and fitting may be necessary.
Once satisfied with the overall fit, our installers begin tack welding the new panel in place, working center out. Of course, this is followed with final welding, and then it's time for bodywork and paint.
When the job's completed, you'll have a big back window cab you can be proud of at a cost of around $750, new window glass and weatherstripping included.
Shown is the donor back window...
Shown is the donor back window clip taken from a '55-59 Chevrolet/GMC half-ton. They are both the same. These clips usually price out at about $250 at your local bone yard.
Preliminary work begins by...
Preliminary work begins by outlining the inner panel cut area inside the cab....
...Note that the cut line...
...Note that the cut line was drawn along the body and door jamb seam lines.
On the outside, Cimbanin runs...
On the outside, Cimbanin runs his cut line on both the upper and lower belt lines just a hair above the stainless steel molding clip-mounting holes.
In order to maintain the cab's...
In order to maintain the cab's inner rear wall conformity prior to doing any cutting, Cimtex first tack welded in a series of three pieces of steel tubing to keep everything in place.
Tim began by cutting out the...
Tim began by cutting out the truck cab's inner wall using a Sears Craftsman electric saw.
Once the interior panels are...
Once the interior panels are cut, it's time to cut the exterior sheetmetal....
...Darrell Cimbanin first...
...Darrell Cimbanin first cut along the bottom trim line, working his way around the circumference until he reached the top of the cab.
Darrell removes the double-wall...
Darrell removes the double-wall small back window panel. Note the lip from the inside of the rear panel.
What we're left with is a...
What we're left with is a nice, big open area that's ready to accept its new big back window.
Darrell trimmed the excess...
Darrell trimmed the excess material from the outside of the donor clip....
...This time, he drew the...
...This time, he drew the cut lines below the stainless steel trim mounting tab holes so the two pieces could be overlapped when they are joined together.
Tim made the final cuts on...
Tim made the final cuts on the donor clip....
...Note the difference between...
...Note the difference between the old back window panel and its "new" replacement.
And in it goes.
Once satisfied with the f...
Once satisfied with the fit,
...the Cimbanins ran a series...
...the Cimbanins ran a series of tack welds starting from the center out,...
...spaced approximately 1...
...spaced approximately 1 inch apart from each other.
Note that they paid special...
Note that they paid special attention to the big back window channel...
...areas to make sure everything...
...areas to make sure everything is exactly where it belongs.
At this juncture, Tim Cimbanin...
At this juncture, Tim Cimbanin began body-working the rear window area using small amounts of body filler....
...Cimtex also relocated the...
...Cimtex also relocated the fuel tank filler.
While the truck was apart,...
While the truck was apart, Tim and Darrell reworked the sheetmetal below the rear window area, reasoning it was far easier to do this now rather than later when the pickup bed was bolted in place.
This is how the finished product...
This is how the finished product looks-ready for new rear window glass and weatherstripping.