Have fun calling Parsons hahaI would think you could mill .080 safely. Again, that's just a guess. You still need to check it. .100 may even be possible. 8000 rpm may not be needed with this size engine. Its more of a torque monster. If running gasoline I would not try and get a compression ratio much higher than 12.5 to 1. I know many have done higher but many have blown them up too. Alky can go higher. Higher than this and the block will split, or is likely to. This size engine needs a ton of flow too. Big carb with big ports and valves. Most of the 460 "clone" type engines come through Vegas Carts and/or Parsons Racing. Give Parsons a call. He will have better insight as to what this thing needs/wants.
This Honda 460 Clone CR Chart I found says if Stock Head was 42cc = 9.25cr and if you Milled off .070" = 11.063cr. Another site said Stock Head was 46cc for 8.2cr.I'm building gx460 back up motor. Highly modified or wild engine. IV got question about milling the head. I took 70 thousands off the head last time with 310 lift cam and have 1.3 roller rockers. I was wondering how much more can I go with same cam and roller rockers?