Gary, I personally wouldn't be affraid to run about 12:1 atleast in this engine with the right camshaft, but if you want to be conservative, keeping it at around 11.5-12:1 should be a happy medium. I have seen flat tappet engines with close to 13:1 run quite well on pump gas without reducing the ignition timing by a dramatic amount. Another tip would be to have your ignition curve done by someone knowledgable with what you are trying to acheive.
AL...