Small roots blower for smallblock

Bought the setup jimjimjimmy had. He gave me a killer deal and is a great guy! Showed up in the mail today. Has a 6" lower pulley and a 3" upper pulley so is 50% overdriven. Based on the online calculators could be anywhere from 7-14psi based on the efficencys of both the engine and supercharger. Thinking i should probably start with a smaller top pulley but was wondering what others with experience thought? I dont mind running a mix of race gas, or the station a few blocks from me sells aviation fuel. What will my boost be?