My 49 had a harsh ride and I discovered I had overload springs on the rear. These were coil springs which anybody with old Chev experience has never seen. Removed them and the ride improved. It is a Canadian built and maybe coils were unique to those. Sure looked like a factory install.
Another probable cause for the vibrations is the bushing in the torque tube. The seal in mine failed and I replaced it with an Okie assembly. I was surprised when I pulled out the original bushing and saw how out of round the hole was in it.
I used teflon on my springs. I found an old "Flying Carpet" snow sled at a flea market and cut strips to fit. They worked but as they did not have edge strips like Posie's (I believe) they eventually wiggled out of place.
I have 650-15 Firestone bias-ply on the truck and they work fine. With the new rubber compound they do not flat spot if unused.
