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

Not sure I understand the question. Are stainless valves softer and cushin' the seat? The seats get beat in because there is no lead to cushin the valve hitting the seat, and it will beat the seat into the head if they are not hardened seats. It's the hardness of the seats that protect the seats.

The cast iron head sticks to the stellite exhaust valve face and acts as a cutter. I have seen Chevies about 3/8 down or more from the top. Good stainless steel does not pick up cast iron on the valve head.