Intake manifold won't seal - How do I check it ?

Ok...typing out loud here...the tabs from the head gaskets that fold down and intake sits on.... cant you put feeler gauges under each 4 corners until the intake bolt holes to cyl head bolt holes look like they line up?? Then once holes are lined up in theory the angles should be the same no?? So "pushing" the intake from one side or the other and measuring that gap...divided in half should give you approx intake gasket thickness.
BUT, in this case you would have to use "thicker cork" intake to black gaskets(in place of RTV??). If such a thing exists.... I dunno Im at a loss and babbling so that hopefully something sensible comes out.....LOL