372or 416 and why

You need more exhaust flow in them heads to start.

Grind out the kink in the roof, take out the end freez plugs and start with the end exh ports.
You can feel 1 finger on roof of port , 1 finger into freeze plug hole on the other side of the roof to tell how much material there is to work with, you don't wanna go to thin or you'll have hot spots, also cut the exhaust guide out completely then grind the exh short turn so it pretty much go's straight up from the valve seat and just smooth the top edge[short turn to floor].

On the 2 center ports, open up the width of the pinched off section[@ port exit] of the floor where the head bolt recess is, but don't go to thin or you'll have gasket sealing issues, and on the end ports open up where the port pinches off on the far ends just a lil.

Some may disagree with this advise but I know it works and would not recommend it if it didn't.

Your exhaust flow is next to stock to be honest.

You will pick up a ton of flow on the exhaust ports if you do these things and these are pretty basic procedures that will add lots of power/rpm/flow ratio ect. to your combo just to start.

How much cranking pressure do you have right now?