milling heads for comp ratio increase

OK, OP, very good summary. Looks like you did some homework on this before. Glad to hear the piston height are indeed level with the deck; that is a lot better than a stock 318! The small displacement of the 273 makes small changes in chambers, etc. a lot more critical to static CR.

I ran your number through with 68 cc chambers, .028" head gasket, and about 4 cc's volume in the piston eyebrows and down to the top ring, and come up with 8.17 static CR.

As for the milling, several folks here have reported that they can mill up to around .030" off the bottom of the heads and not have to mill the intakes and things still line up. So assuming you did that, that would would cut around 5 cc's from the chambers. I'll guess that the flat-faced exhaust valves will save around 1 cc, and if you did that, shaved the heads .030", and found/used a .021" head gasket, then your Static CR would be up a bit over 8.9. That is a pretty good increase for those small changes, and that will definitely show up positively in street performance.

I am now curious about your cam duration specs. Being solid with .470 lift, it might be of pretty long duration and that can become a low rpm torque killer.