I'm doing that right now. It's a real pain because there are other structural members under the floor that are also rotted. I even had to replace the cab support that rests on the frame. I bought an aftermarket floor replacement piece and it didnt't fit very well, so I have had to cut it all up anyway. On the passenger side I think I will get some heavey sheet metal and replace as needed. Mine was so rusted two of the seat hold down bolts broke while removing the seat. I drilled them out and the threads look like they were saved.
Mine is a 1975, and a full convertible so the cab is reinforced under the seats with a steel box that was also rotted. I removed it and welded some structural iron and I hope it is strong enough.
To answer your question, it is a big job, but you can do it. I assume you have a mig welder. I'll send some pictures of the "bodgery" I have done when I finish.
Good luck!
"It's better than a sharp stick in the eye."
