John,
I had the same problem on my '63 3/4 ton. I had to build a bracket and weld it to the frame mounted part of the cab mount, and then use 1/2 mounts. Just grind all the rubber off the metal base and fabricate one. You can probably look at someone's 1/2 ton, take a few measurements, and get some idea of what you need to make. Good luck.
