Josh , I had the same manifold/ head dilemma when I put together the motor for my T . brodix IK 200 heads , older edelbrock street tunnel ram . I think there's at least 4 different intake gasket sizes ,. I ended up with a medium port size that had enough material to come to the top edge of the port & still cover the floor of the opening . Its been awhile , but I had to trim the gasket slightly where it rests on the China walls in order to maximize coverage @ the ports. Its been on now for a number of years without trouble , but it took time measuring ,getting the right gasket ,& also getting proper alignment . the 400 blocks are notorious for leaking between cylinders. Especially 3&5 - 4&6 . I have a machinists gauge block that's large enough to check the deck . We had trouble with the 400 blocks in 1968-72 @ the Chevy garage , the fel- pro composition head gaskets generally work well .