Try this. They have tanks that bolt in with different fill options.
Gas Tanks From Classic Performance Products

Or try LMC
http://www.lmctruck.com/icatalog/ca/full.aspx?Page=83

 

I relocated the tank on my '65 C10 from the cab to the frame. I did a "How To" thread here: Rear Mount Gas Tank 63-72 Short Bed Step Side - ChevyTalk --The Social Network for Chevy Fans

The rear fill tanks do not work well since the license plates on the rear bumper is low.. the gas has to drain down into the tank. I have seen them done though, and they hang down so low they look hideous, IMO.

Here is a thread that shows a variety of different tank installations, including a rear fill style: where is a good GAS TANK thread... - The 1947 - Present Chevrolet & GMC Truck Message Board Network

 

Thanks

Thanks guys! I appreciate the info. I really liked the 49-52 car installation. It looks clean and I can handle the gas cap being in the side of the bed. I had a 65 F250 that I installed a frame rail tank in and made a metal guard to protect the fill hose. This could work the same way.

I looked at the other thread and understand what you mean by the tank hanging too low. It was butt ugly and I don't want to be driving around in the vintage version of a Pinto.