How much do head gaskets compress?

You mean this a baby hemi (331,354,392) or the 426 Hemi? On your questions... When you read figures on head gaskets they typically spec the compressed thickness. The areas with ribs have a little more ability to compress on a steel shim type gasket but the spec won't go any smaller than the un-used gasket. So a steel shim that is .020 will be .020 when it's installed. A composite gasket will be thicker, and usually compress to around 75% of thier original size. The gasket bores are usually designed for the maximum overbore of the largest bore they can fit. So a current 273 gasket will have a bore figure of slightly larger than 4.06. The std Felrpos and stuff are based on a 4.10 bore because you dont want the fire ring overhanging the cylinder where they chamfer the top of the bore for piston/ring installation. Hope that helps. I'm not really a hemi guy but the info would still be applicable.