Everything I have read says maybe 9.5 or 10 to 1 is about the compression limit for street gas. But you can squeeze out a bit more with fuel injection. Maybe 11-1 with fuel injection. However fuel inject brings a whole new set of headaches into the build up. With a blower you proabbly would go a little lower.
Saw a nice 1963 nova at McDonalds this weekend. Had a blown big block. Let me tell you that it was really tight getting it in there. I think water dropped on the top doesn't hit the ground for a couple of weeks. Anyway he said he was running 8.5 to one compression with his motor. Of course he also said he was running pump gas and my friend who should know smelled AV Gas. I didn't have much of a sense of smell, but it sure didn't smell like basic pump gas. Sure was a sweet car though.