I vote for the Powerglide for flatbottoms. I have a builtproff one for my blower motor and a rebuilt, heavy duty one for my ski flat. Never ever had problems with them. 1st gear for around the marina, 2nd gear for driving. Neutral and reverse is real nice. would never go back to an old in/out box. maybe in a daycruiser using a T400 to be able to shift it, but in a flat, really no reason.