Milling heads

I've got a pair of 993 heads for a 350 chevy. I believe the 993 has a 75cc combustion chamber yielding around an 8.5:1 compression ratio with flat top pistons (???). I'm doing some porting work on them and will have a machine shop do a valve job, at which time I'm considering having them mill down the heads to decrease the combustion chamber volume. Will the heads maintain their strength after milling? How much milling to get them down to ~64cc? Anyone had experience, suggestions, comments?
thanks

SORRY, should have done a search before posting. I saw another post that answers my question. Would've deleted this post but it wouldn't let me.