I just did a similar thing mounting a 56 GMC bed to my 88 toyota. No local suppliers of rubber that I could find and the drive over to the wood mill to scrounge used conveyor belt seemed like a hassle, but I did have a 6" piece of left over 2" radiator hose...
Split open I believe it looks just like the original material from my 58 bed and I know its reinforced to boot.
I did have to watch out for the added thickness when body lines are considered but wood blocks are easy to adjust
