X heads on a 273??

There is really no point in doing it

Why, to get more HP, well then drop in a 318 or 360 and use those X heads.

A 318 short block with 11 to 1 pistons would likely be around 10.25 with the X heads and a .050 head gasket, Fel Pro has a head gasket that was like .052 when I check mine. Forgot the part number but summit had them just a few years back.