Getting 325-350 hp from a 389/400 is pretty simple IMO- it can be done with or without ported heads, a few different cams, and the tried-and-true combo of a Holley 4160 and Edelbrock performer. Higher compression 400's can make 375-425 hp, but I would avoid these combos if you want the ability to run lower octane gas and and have a very easy tune.
I recently built a 389 for a 65 GTO, and even with a mild Crower 60241 the 389/400 had plenty of power from 1500-5500 rpms, thanks to the no. 11's that were fitted with 2.11/1.77 stainless valves, port-matched and treated to street port job. The block was bored/ honed .060 over, zero decked, align honed, and compression was set to 8.6:1, with Sealed Power forged 400 pistons and 5140 forged replacement rods installed on a standard 400 nodular crank. Exhaust consists of RARE manifolds hooked to a 2 1/2" Pypes system. The mild cam combined with a mid-sized combustion chamber will still make good dynamic compression and require 91 octane, but running a cam with more duration and LSA will let a 400 tolerate lower octane fuel.
Any late model open-chamber D-port head will work on a 389, but the no. 12,13,48, 62, or 670 heads have smaller 72-75cc chambers ansd will make over 10:1 on a 400, requiring 93 and a careful tune to run- 14cc dished pistons will be needed to drop the SCR down to 9.5:1, the max on 91-93 octane. The plus side to running theses heads is that they come with screw-in studs and 2.11/1.77 valves already. The best bolt-on head that will yield a pump-gas friendly 8.7:1 SCR is a 6x-4- it comes also with 2.11/1.66 valves and screw-in studs, and one of the best stock exhaust ports of any regular production Pontiac head.
Geno
