From my notes gathered off the net
On the small block Chevy, the rule of thumb is, that for every
.005 in. removed from the head or block deck, a hair over .006 in. must be removed from the heads intake face (or the side of the intake manifold)
and between 0.008 and 0.009 in. removed from the end mating surfaces.
IIRC this is the same for the big block.
and my notes copied from
"How to Rebuild Small-Block Chevy LT-1/LT-4 Engines" by Mike Mavrigian.
"Whenever you remove material from the combustin deck surface of the heads, you obviously shorten the head,
which can affect the sealing surface angles between the intake manifold and the cylinder head.
If the resurfacing process removes up to a max. of about .019" you will likely be ok.
If you remove .020" or more from the combustion decks, corrective material removal
from the intake port deck and intake-to-block rails will be nesessary.
Here's a handy reference to use if you need to match the intake angles.
Consider the amount removed from the combustion deck and multiply this by a factor of 1.2
in order to obtain the amount that must be removed from either the intake manifold mating surface of the head
orfrom the intake port deck of the intake manifold.
So, if .010" is removed from the combustion deck, you need to remove .012"
from either the intake side of the head, or from the intake port deck of the manifold.
You also need to remove material from the front and rear rail surfaces of the intak manifold,
where they contact the block (since now the intake will sit to high to allow proper sealing at the port decks).
Based on the amount of material you removed from the cylinder head combustion decks, multiply this by a factor of 1.7
to abtain the amount of removal needed at the intake manifolds front and rear rails.
If you milled .010" from the heads combustion decks, you would need to remove .017" from the intake manifolds front and rear surfaces.
Combustion deck removal amount X 1.2 = Material to remove from intake port side.
Combustion deck removal amoutn X 1.7 = material to remove from intake manifold front and rear rails."
HOW MUCH did they take off the heads.?
Was it a straight cut or angle milled........Are the two port faces still parallel on both heads with the manifold.
My guess is quite a bit. I had the block decked 0.030 (thirty thou) and the manifold still fitted pretty well (at the top)
IF the manifold is sat on the china walls (front and back) then the manifold needs trimming.
......... You __CAN__ cut this down with an angle grinder.
Don't try to do this in 1 cut... keep checking.
It's sealed with silicone anyway so the fit isn't that important.
IF the Port faces are NOT PARALLEL then you will possibly need to reface (angle cut the manifold) You could cut the heads but i wouldn't unless they HAVE been angle milled.
When the ends have been cut down and the port faces only are touching the heads, you can measure the clearance/taper (without gasgets) with some fat plumbers solder across the valley, across the port centrelines. (That's the bit you are interested in.) and bolt the manifold up to crush the solder. You can then measure these to visualise any taper front to back & top to bottom.
Felpro do (did) a guage for measuring the face angles on heads & manifolds. You probably don't need it as the solder method works much better.
I did find it useful to confirm the Edelbrock LT4 manifold angle was wrong but i'd already guessed that.
Fel-Pro Cylinder Head And Intake Manifold Angle Guage 2520 Small Block Chevy