I had a '68 Goat that had been someone's weekend racer. By the time I got it, it had a groove in one cylinder and when you nailed it, left a cloud of oil smoke that blinded everyone behind you. It also had the Hurst Dual-gate shifter, a tricked out automatic, headers, and a 4:88 posi rear. Topped out at about 100, but got there pretty quick.
I learned about the cylinder wall groove when the timing gear let go with it wound tight and bent a bunch of valves and push-rods. Got it fixed at Malcom Durham's Speed Shop (a modestly famous drag racer) for the amazing sum of $300.
Pontiac transitioned from the 389 to the 400 during 1967. They used the same basic block for everything from 287 to 455 cubic inches. Most, alas, had cast-iron cranks and were limited to about 5500 rpms. But, as the guy that sold me a Ram Air III cam for my '69 said, "for what they are, they run pretty good".
In retrospect, I wish I'd spent the money I blew on cars on nice guns, yet another example of "too soon old, and too late smart".