Okay, so I've lost my mind....I had the decks cut .025" on my 400 sbc but I can't remember how to find out how much the intake should be cut. I know the shop should know, but I would like to know also...just for my own peace of mind.

 

If you're running 0.038" or thicker head gaskets, with un-milled heads, you should be fine. Stock 400 head gaskets were steel shim, NO thicker than 0.020".

 

I used Fel-Pro permatorques, 39 thick. But there is a good gap between the block end rails and the intake (Performer RPM intake, World Sportsman II heads). WTF? How do I know the intake ports are sealed at the bottoms?

How do you measure from crank center to deck?

 

If the gap is about 1/16", it's okay. If it's more like 1/8", then something's up. Offset dowel pins might be the cheapest and easiest fix, but then you have to re-check your valve clearances. Especially since those heads really respond well to bigger valves. Not just piston-to-valve, but now valve-to-bore, also. And then you have to check your pushrod clearances.
To check the deck height, you gotta get a vernier caliper that'll measure 8"+, then add half the main bore to the bore-to-deck. Not super accurate, but close enough.

 

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Milling the intake lets you transfer the heads to another block, but considering the crap heads, better to mill their intake faces so you can try other intake manifolds. Besides, it's far easier to mill the heads than the manifold. Less labor should be cheaper.

 

This should help you out. Found it while working on my project. http://enginedev.blogspot.com

 

I would bolt the heads and then put the intake in place and see how it fits. Mainly how the ports line up between the intake and heads. Then go from there. You may not have to do anything.

 

Okay, I looked deeper into it...much deeper haha.! Using a bore scope I found that the intake gaskets (port rails) are too low thus causing a blip in the flow. The intake lines up very well fore aft and up down. The end rails gap is < 1/8". I will remove the intake and reposition the gaskets and move on. Thanks for the help.