Just a thought, but I had about the same issue when buying my '73. It turned out the original GM tank had been replaced with a custom-made job, so the tank didn't match up exactly because of that.
I've since replaced the fuel tank with an original GM part and everything lined up as it should. Now, though, the new tank has also "drifted" maybe half an inch or more out of alignment, so it would seem there's some normal leeway that shouldn't be worried about. (If I recall correctly, the way the tank is fastened probably allows it to move slightly.)
Somebody more knowledgeable can probably provide you with better information soon enough, but I wouldn't immediately suspect frame damage because of this.
If you have doubts about the frame, you should check that both the doors close properly, that the car seems symmetrical from different angles and vantage points (use measuring tape if necessary), that there aren't stress-induced cracks in the front window or in the fiberglass body, that both T-tops fit as they should and properly remove and replace, and that there aren't any obvious weld marks on the frame itself.
Regards,
Arto
