want to chg fuel tank side fill on a 72 elcamino to a rear bumper fill car fuel tank. is that a real problem.

That can be done, and as far as any issues would depend on your abilities, tools, equipment ect.
http://i300.photobucket.com/albums/nn18/coyoteon2/tank201-2-1.jpg
It's not what you would call a direct replacement if that's your question.

By the way, WELCOME to E.C.C.