I used 360 heads on a 318 that had too high of compression with fel pro .040" gaskets. It brought the compression down to 9.2
I would think that you would be somewhere in the 8.8 - 9.0 range without doing the calculation myself. You have a couple of different variables that would be difficult for me to calculate.
What is the current cc of your original heads?
What is the stock head gasket thickness and diameter of the fire ring of the head gasket for the 10.5 pistons and head cc's.
Also, you will have to notch the bores to get the larger valves in the 273. Are you going to do that?
I can do the compression calculation if you provide the information that I need and are going to notch the bores. I don't want to go through all of that if you are not going to go through with it. I am willing to do it if you are serious about it and can give me the missing information.
What is your target compression? I can do the calculation backwards and tell you how much clearance volume you will need to go from 10.5 to whatever your target is.