The bigger problem is to find out how much the heads have been milled, if you can find that out the formula is 1.2 to 1 on the intake mounting surfaces for every .001 removed from the heads
Have you looked at setting the manifold in place without the gaskets to see if the ports line up without the gaskets?
You may also have to mill the bottom of the manifold after you mill the sides.
Without knowing how much the heads have been milled it's kinda touch and go, also keep in mind the gaskets will get thinner by about 25% when the manifold is torqued down, if you're using paper gaskets.
JMO
Zip.