On the GM multi leaf springs I've done, there have only been rubber isolators right in the center of the springs on either side, just as 69Z28-RS describes, and the end of the leaf has plastic anti squeak pads along with your clamps and small rubber for those.
Ultra High Molecular Weight Polyethylene or UHMW is pretty cool stuff, comes in a roll, and could work as a substitute on the ends of the leaf if you don't want to buy the spring refurbish kit with anti squeak pads, but I can tell you the UHMW costs 3 times more than a spring refurbish kit does. I'm not aware if UHMW comes thick enough to substitute for the center rubber isolators. Those are fairly thick.
I'll be rebuilding my rear springs on the Z, and just plan to use the simple rebuild kit, probably try Heartbeat. I'd guess the kits are similar. I did a 61 vette a few years back and used what Corvette Central had, worked fine, and did a 67 mustang and a 65 Comet last year and used what NPD had and replaced all hardware as well, which was a mixture of Scott Drake and AMK. Worked excellent.