PTO's all run at a given percentage of engine RPM and some are quite low, i.e, 32% for example. Assuming your transmission is an SM420 or original 4 speed a typical single gear PTO runs at approx. 65% and they don't like more than about 1000-1200 RPM so your engine speed could not exceed about 2000. That isn't enough to drive the truck and go anywhere. Many of the old single gears had bushings in lieu of bearings. Hard telling what you have however. Also it must be engine rotation to operate your AC compressor.
So the advice you have already gotten is correct.
