what heads to run on a 273

What dome size? The "302" head chambers are small or also known as closed. A dome very well might have interference with them. Double check with Egge. The piston maker right? Or who ever made the slug.

As far as intakes go, there are 2 different intakes for the early engines. The 273 often takes a odd angled intake attaching bolt as well as a smaller size @ 5/16 instead of the normal 3/8.

(Unless the year of the head is different, like a newer 302 head is used of course.)

Then, any other 66 and up 318, 340/360 intake can be used.

The Egge HP pistons won't give you much more than 9:1 without milling the heads and using a thin head gasket.