I think that with a smaller engine you could get away with 9:1 compression on 93 octane pump gas. Unless you have a LP head, you arnt going to get that compression with off the shelf pistons.
If you're going to get custom pistons then there's no reason to mill the head any more than necessary to get it flat.
Milling the head will not gain you much, and with an off the shelf piston, it might be the difference between 7 and 7.5 compression. Cam timming will show more of a difference on a compression tester.
As far as off the shelf pistons go, I really have no idea which ones are the best.
For custom, have you tried the Farmall builders? I think that Baker or Lemmons should be able to build pistons based on your head casting number. They should know the cc's already.
I don't know for sure, but I think that ALL the custom builders use Arias or Ross for smaller pistons like that. Difference is that the farmall guys should already know how big of a bump to put on top.