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My perception is that you need something there, though I'm willing to accept correction about this. The other reason that I stuck with the original rubber was that I was told the polys ride harder and I figured the stock 1985 Z51 was hard enough ...

I don't want to discourage you, but 150k is really high mileage for serious C4 judging. There's a likelihood that many of your original parts have been replaced and there's a lot of preservation (cars with say, under 20k, often trailered) and not so much restoration in this part of the hobby. I stood out from the rest of the C4s when I got my final Top Flight at about 69k.