8mm because of the fact you can change the blower by 2% easily without changing belt lengths.
13 to 1 ? Not sure I would go there. 14/71? Fresh? I have to wonder just what "job" you are looking for this boat to do.
I'd certainly be underdrive. Too many variables to guess at lbs of boost.
I would guess that I would put the blower 10 under to start.