Magnum head swap compression increase

IMHO, that is the largest I would go. (But I am a low RPM torque junkie LOL) DCR numbers look to be in the low 7's and that is pretty good for that old low compression block and any sort of camming up.

BTW, if you go with those 1121G head gaskets, then check the heads and deck for flatness; those thin composition gaskets reportedly are more prone to leaks, and I suspect that is due to folks slapping them into engines with slightly warped head and or deck surfaces. So skimming the heads at the shop would be a good idea for this head gasket IMHO.

Or, if it is just slapping things together, then you could just go for it and see if it holds!