Try setting your intake onto the heads and block with no gaskets or end seals and use a feeler gauge to se if you can fit a .0020 to what ever size will fit in between the manifold and heads on all four corners and at the bottom and top of each corner and write the results down
if you end up with different readings on all four corners top and bottom like left rear bottom takes a .005 gauge and the top takes a .0025 and the left front top takes a .007 and the left front bottom takes a .010 and the passenger side ends up with similar readings you will need to have the intake machined to get it fit into heads so it is fitting equally on both sides with no gaskets.
I like to have the bottom of the intake tighter by .0010 to .0030 tighter than the tops so the manifold will pinch the gasket like a V at the bottom
Another one of the several reasons I call the 426 Street Hemi motors "Hemiroids"