[b-man]

Lower the compression using dished pistons.

These engines were advertised as having 10.75:1 compression but were actually about a half point lower. I’d recommend dropping down to around 9 to maybe 9.5:1. A manual trans car can get by with more compression than an automatic car, also rear end gears are a factor as well. Higher rear end gears numerically will be happier with more compression, conversely if using taller highway (lower numerically) gears you’ll want to err on the lower end compression-wise.

Upgrading the cam to the the factory 068 1965 - 1966 360 hp Tri-Power cam or perhaps the Summit 2801 will give it a nice boost over the original 009 cam that was used in 1964 on both 4-barrel 325 hp and Tri-Power 348 hp engines. The 009 cam is the same profile as the 067 cam that was used in 1965 - 1966 4-barrel 335 hp GTO engines.

If you’re not spending the money to convert the heads to screw-in studs which won’t be cheap I would suggest sticking with the factory 068 cam. However after inspecting the pushrod guide slots that are machined into the head castings and finding some to be excessively worn then you should resign yourself to converting to screw-in studs and aftermarket adjustable guide plates. Once those guide slots are worn out you really have no other choice if you want to keep using the original 1964. 9770716 GTO heads.

The 1967 Ram Air / HO exhaust manifolds will buy you maybe 8 horsepower at most over the standard log manifolds so for the additional 500 dollars there’s not a lot of bang for the buck on a near stock 389 engine. A nice set of 2-1/2” PYPES headpipes and whatever mandrel-bent 2-1/2” exhaust you like will be plenty for a mildly modified 389.