They had compression as low as 7.5 to 1 on some engines in the mid to late 70's
. I believe they were expecting even worse gas than they wound up with. But I think 9.5 to 1 should be fine with 93 octane if you have a good cooling system, and you got a good tune on the engine. An old "rule of thumb" used to be run the same compression as one tenth of the octane you were going to use, but there are still a lot of variables. Examples-8.9 to 1 compression for 89 octane, 9.3 to one for 93, etc. I have ran about 10.0 to 1 for years, with a mild cam, but I have a good cooling system, and I still have to mix in some octane booster, and have my "total" timing "locked out" at about 32 degrees. Im getting sick of this after about 24 years, as gas seems to be getting even worse, so Im having Mr P. Body put me a 455 together meant to run on 89 octane. He is using a slightly higher compression ratio than 8.9 though. Cam duration affects this as well, and I beleive you can get away with a little more on 400s than 455s (less swept volume)(stroke). Hopefully one of the "builders" will chime in tommarow to shed more light on the subject.
Just offering up some ideas. Good luck with your build, and dont forget to install the best parts your budget can allow in the short block for years of troublefree service.