Will 318, 340 or 360 heads fit on a 273?

I typically skip hardened seats and replace the exhaust valves with stainless steel. No problem so far. The exhaust valves usually need replacing anyway. Use what you have. He does not need much more than what he has.
I've seen exhaust seats beat 1/16 into the heads... just say'n. that's why I prefer hardened exhaust seats.