replacing heads on my 79 TA

[fullmoon]

I recently found a set of heads from a 1971 455 casting #66. What should I expect if I put these heads on my 1979 6.6 400. on the 455 it was rated at 325 hp the valves are 2.11/1.77 114cc?

[Morley]

the only real number advantage those heads have is a slightly larger exhaust valve, 1.77" vs your 1.66" They had 114cc chambers and your heads have either 98 ir 101 cc's. This means that you will loose compression using the #66 head on your engine. You could try using a thinner head gasket to make up the difference.
A better factory head would be the 1969 R/A IV head, which had the same size valves as the 455 but had only 72cc chambers, which would raise your compression.
As a note your stock compression was only 8.1:1, good for a highly supercharged engine, very poor for an N/A engine.

[MrPbody]

Agreed. 455 heads are NOT desirable on 400s. The only exception to that would be the '70 455-HO, which used casting no. 64. Rare as hen's teeth today. That was the ONLY "high compression" 455 offered, and the only "HO" that used D-port heads ('76 doesn't count, as that engine was "HO" in name ONLY).

You ARE certain you have 400 Pontiac, right? And not 403 Olds? If the scoop has "6.6 Litre" on it, chances are it's the Olds. If it has "T/A 6.6", it's the Pontiac.

If it IS the Pontiac, the 6X heads there have tremendous potential. Installing the larger exhaust valve is a matter of routine for a Pontiac shop. The intake runner is a very good design, and the heads can be ported to flow in excess of 270 CFM @ .550" lift. The chamber for the 6X-8 is 98 and the -4 would be 94. These numbers are "nominal", as each set varies a bit. One must actually measure the chambers to be sure.

You will pick up more power by changing the cam over any other single modification. For the lower compression engines (under 10:1), we recommend Comp XE grinds. Haven't found anything better to date.

Get Jim Hand's "How to Build Max-performance Pontiac V8s", published by SA Designs. TONS of CURRENT information there! All aimed at high performance STREET engines. You'll find making big power with a Pontiac is not nearly as difficult as the "bowtie" boys want you to think!

Jim