I don't ever remember hearing of a problem with a gas tank behind the seat. Most people who remove them want more cab space.
Just make sure that the gaskets are in good shape to prevent fuel smell in the cab.
Whenever I have purchased an old truck this issue has never crossed my mind. I guess I put a lot of faith in GM's original design and engineering. Also, I like the easy access to the fuel tank in case I need to work on it, it's better than crawling under the truck!
