383 knock goes away on warmup

Most likely piston slap.

Wrist pin noise [ worn pin &/or pin bore ] gives a double tap.

As long as the rest of the ign curve is altered to suit, running 20* initial is a good idea. Engine will run cooler, use less fuel & have better tip in response. The GM LS engines have a very good combustion chamber, only need about 27-28* @ WOT. They idle with 22*.....