.40 over 170 rebuild

I didn't say increasing the c.r. Would be a waste, I was trying to make the point that milling the head prob won't get you enough of a reduction in chamber size to matter. And you could weaken the deck, there is water(coolant passages) all over the place in the head.

If you want to bump the compression without changing parts MAYBE milling the block would get you partway there but there are limits to how much you'd want to do, as too much will put you into the water jackets and it's likely been milled already.

Those things are what led me to the the idea of offset grinding the crank, putting the piston up higher is the preferred way to get your target c.r.

You need to figure out where it's at before you decide where to go, you could have nice compression in your free motor already. Then it's just a matter of cam selection etc.