The H75 head has a big cylindrical dish around the combustion chamber, I can calculate it's capacity but that'll have to happen tomorrow.

I've recently gotten rid of my race GX head in favour of a smaller port (but ported) H75 head. It has been machined to remove the large dish so the combustion chamber shape is similar to that of a GX head. Machining this off brings the comp ratio right up, I can't help you with calculating what you'll end up with until you give some more details over; pistons dish, or fly-cut cc; piston height @TDC relative to deck height; head gasket type (standard supplier or thinner custom item), bore size, stroke, comb chamber cc etc, valves (stock or dished type or flat-top) etc etc

10.5:1 is perfect and should behave on most pump fuels (slightly dependent on spark timing) unless you get some real low grade stuff.

Flywheel? I can't remember the spec of mine. You don't really lose power but I get the inertia is somewhat affected. I have a little trouble with my motor getting off the mark, especially on a hill, everything is lightened and the cam doesn't wake up until about 3500-4000 so a hard start is somewhat sluggish with the cam and lightened internals + flywheel. Look up user name lemonhead and have a look at one of his recent posts where he talks about his flywheel, pretty sure it's on his race car but worth a look as I think he details it's weight.