1993 Lincoln Mark VIII 4.6 DOHC -- 9.8
1996 Camaro LT1 -- 10.5
I have had both of these and run 87 octane (Oregon gas sucks) and almost never heard them rattle. Keep in mind that these both had computers to control timing. Both of these cars are recommended, by their manufacturers, to use premium fuel.
By the time you take into account the thickness of the head gaskets (which can vary greatly by application) and the valve reliefs/dish in the pistons, a jump from 64 to 56cc isn't that huge. I'd be really surprised if it puts you past 9.5:1. The cam profile (specifically overlap) will probably affect the need for specific octane requirements more than anything else. Your new heads should be fine. If you are really concerned you could order thicker head gaskets.