I have a '73 J head 455, standard and unmolested.

I'm pulling the heads off to replace with C heads and have a Felpro rebuild gasket set. I believe the head gaskets are .042".
I can get a set of Cometic head gaskets that are .027"

What head gasket thickness did the factory use?
What would the difference in compression be between the two? Would the cometic gaskets be worth half a compression point, ie 8.5:1 to 9.0:1?

 

Actually, your 73 motor has the soup bowl sized dishes in the pistons. The result is that the head gasket volume comprises a smaller percentage of the total combustion chamber volume, and thus the thinner gasket makes a proportionately smaller difference. I ran a quick and dirty CR calc and it looks like the thinner gasket probably bumps CR up by at most 1/4 point on that motor. Again, for a higher compression motor with small dishes or flattops, that change would be a lot closer to half a point since the gasket volume comprises a larger percentage of the total chamber volume.

Edit: I should clarify. The stock Olds steel shim head gasket has a compressed thickness of about 0.017". Either gasket is thicker than that. The 0.042" gasket will LOSE about a quarter point of CR.

 

Okay thanks Joe.
Looks like I've some simple math to do to pick pistons that will get me to around 9.5:1 with the Felpro head gaskets and around .004" off the heads. I'm guessing 9.75-10.0:1 dish or thereabouts will suffice.
I don't intend to cc the heads, it's not that important. Just want to improve on the crap '73 factory compression.

On a similar note, are the Felpro .042" head gaskets going to upset the lifter/ pushrod/ rocker arm clearances, or should the hydraulic lifters take up the slack?