The best thing you can do to keep temps down is to do like Cliff says.

I would deck and square the block and set it up for zero deck height. It is not a step to be left out in my opinion. I've seen factory 400 blocks off .017" from one end to the other, and nearly that far off from one bank to the other. The added compression from the decking process will be offset by establishing a greatly improved quench distance. This alone will reduce the octane requirements of the engine, all else being equal.

Pontiac engines in particular do not fair well with the pistons way below the deck. The common denominator for every single running hot/overheating/pinging/detonating Pontiac engine we've been asked to help out has been pistons WAY below the deck at TDC.

We set all of ours at exactly zero deck height, and use a .039" gasket. With the right cam there will be less tendencies for detonation, running hot/overheating than having the pistons .020 to .050" below the holes at TDC with nearly a point less SCR.

Cripe, we've had 400's in here in the low 8 to 1 CR range that ran over 200 degrees at speed and the temps creeped up to nearly 220 idling at stop lights. Pull them down and you are going to find that someone used cast 8 valve relief pistons .050" down in the holes at TDC!

You can build a 400 at 10 to 1 SCR with zero deck clearance making 150 MORE hp than the scenario above and you'll have trouble getting it to warm up to temp's.

Cliff
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73 Ventura, 455, KRE "D" ports, HR cam, RPM intake, 1977 Q-jet, TH-400, 10" Converter, 3.42's, 11.30's @ 120mph in full street trim on DOT's, 89 octane, 3740lbs.