PJ:
The '65 rear cab mounts are a common problem. Chevy Duty, LMC Truck and a number of other aftermarket truck suppliers sell rear cab support replacements. I recommend getting the OEM/GM factory originals. They also sell a "slip-on" replacements, which I would not use since there seems to be some rust damage that is structural. Additionally, depending on the location of the holes, I would repair them. The frame on a '65 has a number of holes when it comes from the factory for mounting various assemblies, ie brake lines, wire clips, exhaust hangers etc. Drilling additional holes, may or may not diminish the structural integrity of the frame, however any holes due to rust must be repaired. I have infilled by welding some of the extra holes in the frame of my '66 just to give it a cleaner look. If you can weld and you have the equipment weld them up. If not, I would take to a local weld shop to have them infilled by welding. Best of luck.
