Magnum heads don't fit a 273 with out a big bore notch or smaller valves and seats installed so not gonna work very well.

318 magnums need about .030 off the deck to get the pistons close enough to the top of the deck for quench with thin mopar head gaskets. Combine that with the factory .049 head gaskets and you get terrible quench.

318 magnums will make ALMOST as much power as a 360 magnum. The 318 had a better cam, more compression, and a flat top piston for better flame travel.