Help! Intake manifold won't fit!

Garrett Ellison said
It sounds like your old intake has been milled sometime before because of a possible head or block mill. Put the iron one on with no gaskets and then measure with two sets of feeler gauges on each side. Then do the same with the aluminum intake and see how much difference you have. And here is a good read for some other things to check and be aware of.
http://www.hughesengines.com/Upload/TechArticles/INTAKE_MANIFOLD_SEALINGJULY2015.pdf

yellow rose said
You need to set the new intake on without gaskets. Look and see if the bolt holes line up.

If they do, measure whatever intake gasket you are going to use (not the steel shims gaskets I hope...they are hard to seal in a bimetal application) and that is how much you mill off each side of the manifold.

You also need to mill some off the end rails of the manifold.

DO NOT slot the holes. You’ll get the bolts in, but the ports won’t line up.

I had the same problem with my Edelbrock intake, but just worked at getting it solidly settled for a bit, and even pushed down on it while putting the bolts in and finally got it.
That was with standard blue Felpro Print O seals.