I have used 72s and 83s.
You can get away with 72s, but you should always go one step bigger incase you upgrade the turbo later.
With my aluminum headed, roller motor and the stealth 70
I was getting about 75% Duty Cycle @ ~ 21psi.
On Kill I think that I would have been in the high 80s.
The motor is good for about 130 plus mph in a full weight car.