Those heads listed are way too big for a 400 chamber wise.

All accessories will cross over.

The 6X, 5C and 4X are 2.11/1.66.

Personally, I feel you should look elsewhere for an engine rebuilder. Those core numbers you provided for heads and block are high production/availability and not that great for a 400 or for strength.

I would look for 67-71 400 block, even lower cost Y and X coded motors which are just as good. 67-71 400 rebuildable core with caps, crank and rods $100-400.

Would also look for #13 or #62 heads (cores $200-400) that have a verified chamber CC of 78-82. Or you could find a set of 1970 #64 heads for around $700-800.

A full service machine shop will be able to build you a motor for roughly $2000-3500 with your core. The price difference is due to many variables in quality of internal parts and options. I would also stick to a Pontiac only engine builder. These motors are quite different and you do not want a Chevy of Ford profile cam with a specific RPM in a Pontiac. As well, the motor should be built around the rear gearing, weight and exhaust..not the other way around.

My must haves for a stock motor

1. Have rotating assembly balanced
2. Can use stock rods, upgrade to ARP bolts.
3. Torque plate bore and hone.
4. oil galley plugs.
5. SERDI valve job
6. Comp Cams valvetrain.

There is a lot to answer here. The head/displacement you are looking at would be a dismal performance of 7.8-8.0 to 1..That is unless they (engine place) spent a lot of time and money milling those heads.

And you would be very disappointed at the dollars spent and result. Personally, spending another $500-1000 for excellent heads and a better block and corresponding machine work would make you happier.

P.S. See you are in Cali.. Steer way clear of Bruce Fulper and Rock and Roll Engineering.