I had a 74 142 that had a similar problem, it was severe enough rust that I had to drop the gas tank and reconstruct the subframe rail which was rusted through - certainly what any sensible mechanic would have declared 'not fixable.' I suppose anything's repairable if you have enough time and patience, but take my advice and DON'T start reconstructing anything until you've got the seats & carpet out, the car up on jackstands, and a clear idea of how bad it is. Otherwise it becomes an all-consuming project with a life of its own... Fix the floor, then find out the rockers are full of Bondo, the door bottoms are fibreglass, the trunk's paper-thin, etc, etc, etc... I've had a handful of those projects which have spiralled out of control - just becomes a chore, no fun.
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Chris, Dartmouth NS Canada 70 M-B 280SE, 83 245DL, 84 244 turbo, 90 780 turbo, 92 VW Golf, 90 740 Rex/Regina