In many vintage trucks, the stock fuel tank was located behind the seat, inside the cab. While this was a very common practice for a very long time from most manufacturers, it never really made sense. Locating the fuel tank under the bed and behind the rear axle is a more suitable location for many contemporary truck builders. This location provides better weight distribution and gets the smell of gas out of the cab, which are two definite advantages. On later trucks this location might interfere with the stock positioning of the spare tire, but on most older, vintage trucks the spare tire was mounted on the bedside anyway. Oh well, whatever the reason you choose to move the gas tank is up to you, just be sure that you do it correctly and most importantly, safely.
On my '68 Chevy C10 the previous owner had replaced the gas tank, but did it with an OEM-style tank. Not only does this take up room in the cab, but if it's not vented correctly the fuel smell lingers in the cab. Additionally, with the 400-horse stroker motor under the hood a rather noisy electric fuel pump had been installed just under and outside of the cab. Perhaps I'm mellowing (or maybe getting soft) in my old age, but I wanted to get the excess noise and the gas smell out of the passenger compartment.
The first step is determining what size tank will fit into the proposed location. Depending on your truck, there may be N.O.S. or reproduction optional auxiliary tanks, or you may need to find a universal tank that fits your requirements. For my '68 Stepside, I chose a universal pickup tank (PN TAN-UT) manufactured by Tanks, Inc. and sold through Yogi's. The tank has a 14-gallon capacity, fits real well where I want to mount it, and includes a rollover vent valve that will stop the flow of fuel to the engine should the truck get upside down.
Universal mounting straps are also included with the tank, but I chose to use some beefier strap steel that I had on hand. In either case, you will need to determine how high or low the tank should be mounted in the truck's frame and bend the straps accordingly. I had the bed off the truck for some unrelated bodywork, making the gas tank installation easier, but you can probably leave the bed on to install a new tank. Since the bed was off during the installation I measured the height of the bed's cross sill to determine how much room would be available above the tank, yet remain underneath the bottom of the bed wood. With that information, I supported the tank on a floor jack at the desired location and measured the necessary dimensions for the two mounting straps that would support the tank between the two rearmost crossmembers of the chassis. The strap material was then taken to Morfab Customs so that the strap material (11/2x1/8-inch thick) could be bent in a sheetmetal brake for nice, crisp bends. The straps could be bent in a vise if necessary, but those results may leave a bit to be desired.
With the gas tank in the stock...
With the gas tank in the stock location, the electric fuel pump is currently mounted just beneath and outside of the cab. Be sure to drain the existing fuel tank before attempting to move it and have some method to catch fuel in the fuel lines.
Back in the garage, I positioned the mounting straps between the two crossmembers so that they would align laterally with the respective grooves in the tank. I then set the tank in place and double-checked my measurements. Locations for 3/8-inch mounting bolts were then determined and center-punched toward the forward and rearward ends of the mounting straps. The mounting straps were intentionally left long on the ends, but will be trimmed later. Although the gas tank will not bounce out while the bed is on, I chose to add a top strap that mounts to each mounting strap above the tank to minimize vertical movement of the tank since the tank is not physically attached to the mounting straps. Before trimming the mounting straps, I used one to mount the rollover vent valve, which should be mounted in a vertical orientation and as high as possible above the tank. The rollover vent valve connects to the tank via a length of rubber fuel hose.
While the bed does not have...
While the bed does not have to be removed to install the new tank, you must compensate for it when determining the correct height of the tank. The filler neck will eventually protrude through the bed wood floor. The wood strips shown in the photo represent the height of the cross sills and the bed wood floor. When finished, the flange around the filler neck will be mounted to the bed wood with the filler cap slightly below the surface.
After mocking up the desired...
After mocking up the desired location, measurements for the mounting straps were taken. To make nice, clean bends in the mounting straps I took the mounting strap material to Morfab Customs so that it could be bent in a sheetmetal brake.
The mounting straps can now...
The mounting straps can now be set into position, that is, between the two rearmost chassis crossmembers. Any excess length of the mounting strips will eventually be cut off.
Set the tank in place so that...
Set the tank in place so that the mounting straps seat themselves in the indentions in the tank. Center the tank between the framerails and adjust the mounting straps for the best fit. Then mark the locations for bolts to be installed.
Use a center punch to mark...
Use a center punch to mark the holes and then drill the holes for mounting bolts to the appropriate size for the hardware you will be using.
I chose to insert and tighten...
I chose to insert and tighten the mounting bolts at the rear crossmember first...
...then reinstall the tank...
...then reinstall the tank and determine the mounting location for the front end of the mounting straps. Center-punch and drill the holes for the appropriate sized hardware. I chose to use the excess from the passenger-side strap as a mounting bracket for the rollover vent valve. The driver-side mounting strap was trimmed to length.
To prevent the tank from moving...
To prevent the tank from moving vertically, I used additional mounting strap material to create two top straps. Those were bent approximately 90 degrees on each end, holes were drilled, and secured with 1/4-20 bolts and lock nuts.
The forward end of the passenger-side...
The forward end of the passenger-side mounting strap was used as a mounting bracket for the rollover vent valve. A section of rubber hose will connect the barbed end of the rollover vent valve to a fitting on the top side of the tank.