Following the above commets, I put myself and 1975 4/4 through a complete rebuild in the '90's. If you are finding rot in the chassis, there is more than a chance that the wooden rocker boards are suffering if you have trim covering them. I don't know if your car had been rebuilt previously? Mine hadn;t, but when these cars wee built in 1975, there was no cuprinol trearment for the frame and the inner wings were in basic primer, before they sprayed the topcoat over the wing piping. Due to the way they were built at this time, many of these cars are quite badly corroded once you start to investigate, as in, it is unlikely to be better than you expected! Good luck.