Yes the 64 has a very solid quarters, doors and fenders. The RH fender needs a small patch, but that's about it. The floors are starting to get some rust though. The car was lowered at some point in time and that isn't helping things.
As far as the signature getting bigger..... when I went to pick up my 409 block yesterday (STILL CAN'T BELIEVE I FINALLY HAVE ONE!!!) the seller had a lot of nice cars including a 61 Impala bubbletop "parts car". It had rotten floors, trunk, the lip all the way around the trunk, and who know what all else, but the glass, quarters, doors, and roof looked decent. He said he'll talk to his father who owns it and see if he'll sell and that he thinks he'll let it go for about $800. It's too far gone for me to make nice, but it would make a cheap / fun drag car with some sheetmetal floor pans and trunk pans (not replacement parts....just flat sheetmetal to keep it simple/cheap) and some lightweight pin-on fiberglass parts. I can't get it right now if he will sell it, but I'm still excited about it.