Wow, someone looked at my page. Honestly I would say anything is repairable. The question is whether its worth the time and money to repair, over just starting fresh with a clean shell. I chose to start with the "rust-freeist" car I could find, and part out the rustier car I had. Looking back, I think it was a good move.
Floor pans by themselves shouldn't be that hard or expensive to replace. Front frame rails are a heck of a lot more work, because of the importance of getting them perfectly parallel, and in all the right places. If you were going with a fully adjustable suspension design you could get away with being less exact on the front rails, but with the stock Z design, there is almost ZERO adjustability, so you must be perfect.
I was quoted 16 hours labour for each of the front rails from a very good body shop. They told me when they were done I wouldn't know the difference from stock by looking at it, and neither would anyone else. They were also willing to throw a guarentee behind their work. Too rich for my blood though.
Personally I would buy a wire feed welder, and do the floorpans myself, and learn. Then after you should have a better idea whether you are up to the task of doing the rails or not.