I have two 302's with GT40 iron heads, one has stock bolt down rockers with stock length pushrods for a 79 engine, and one with stock lenth pushrods with Comp Cams studs and roller tip adjustable rockers. Both have flat tappet hydraulic cams.
On the bolt down rocker motor, I tried the Crane shim kit, but did not need it. Spins to 6 k anytme.