I had the same problem on my 74. I put in some poly mounts and had no problem with them, but then again that truck rode like crap, really stiff. I couln't tell the diff between the old worn out mounts and the new ones as far as ride quality. It did re-align my cabs butt and stopped a bunch of creaks and squeaks. If you put poly mounts on I would recommend greasing them up on the mounting surfaces, I've heard that they squeak after awhile if left un-lubed.
I need to replace the mounts on my '86, but this time I think I'm going with a set of OE rubber ones from LMC or ChevyDuty. Mostly because I want this truck to ride like a Caddy and also you can get all the hardware with their kits and my hardware is in real bad shape corrosion wise. /forums/images/graemlins/waytogo.gif