I am looking at the ground up kit conversion and i can choose from 9 in booster and 11inch booster... the only problem with that is that i have no idea what the difference size means or which size i should get. help!!!

I got the 9" just to have a little more clearance around the valve covers. I'll be interested to hear if anyone knows of a performance difference.

The larger booster will supply more assist. Some smaller boosters have dual diaphragms to increase the amount of assist they provide.
Mike

The 71 had an 11 inch from the factory if that helps (of course only on cars with power brakes)...

This may help.