2 critical areas (1)the car body 63-66 A bodies are the lightest then 67-69 A bodies & 70 & up they got progressively heavier by leaps & bounds. (2)is proper quench so you can run a HIGHER CR without pinging which on a /6 means piston/rod/ head & deck milling changes & adjustments & as mentioned Doug Dutra is THE master & a real nice guy & knows all the tricks in addition to quench that will benefit you.