A 7 inch booster is standard hot rod fare these days. Most vehicles don't have the room for a 12" booster as you specify, I would have no issue of using the 7" booster. The amount of vacuum the engine has comes into play as well.

1946 GMC Pickup - S-10 Frame, 455 Buick, TH400, original patina.

My 46 GMC on Photobucket