I took all the parts I could use from an old 1978 motorhome based on a 2 ton chevy van and used them on my '55. One of those parts was a big-[censored] 35+ gallon fuel tank. Out came the leaker from behind the front seat. My tank is mounted behind the rear axle. The tank is supported using the stock straps that came with the tank. For the front support, I welded in a piece of u-channel to the frame and then attached the straps. At the back, I used the stock location of an existing piece of u-channel. I fabbed up special brackets out of 1/4" thick plate to keep the tank from moving around. I also had to notch the underside of a rather beefy crossmember to provide clearance for the a slight dome on the gas tank where the sender resides. I am using the stock fuel pump which has a return line. I ran new 3/8" supply tubing and 1/4" (I think) return tubing. These lines run along the inside of the r.s. frame rail. I still haven't figured out the filler spout arrangement (box isn't on the truck yet). I should be able to go 700+ miles on a tank of gas at cruising speed.

The tank fits nicely and I have enough clearance on either side for my dual exhause system.