Magnum Heads

Glad you caught that.
Agree 100%!
I know some aftermarket along with the r/t do have less flange around the end ports.
I had bought some Summit shorty headers for my truck that I just put some aftermarket Magnums on 'not even gasket matched on the exh side'.. to find that there was only about .030 width of sealing surface at the lower corners. In fact...I had to weld up the bottoms of all header ports and the lower corners and file flat to make them seal up... I also die ground out all the welding that protruded into the port window they were .150 too small all around!!! ..and when I got done...it finally felt like I put headers on. Avoid summit headers. They are total ****. Just look them over with a mic and good header gaskets...they are garbage.
What a pain in the *** it was to fix them. I would have returned them if I hadn't already modified my down pipes to fit!

Bringing it back...
..So be careful which exh manifolds you use and with what casting heads.

My final solution will be the Doug's D453 headers at some point. Pictures I have seen make it look like they match up well.

I have thought about a Jeep Magnum manifold on the right side and filling the bottom of the port on left side free flow, but no idea what you could fill cast iron with that would stick and stay at those temps.