The rule of thumb that I go by is .250 bigger on the primary tube size than the exhaust valve size. So, if you are running a 1.81 exhaust valve then a 2" primary would be about ideal.

Also, I would run a 3.5" collector even if you run a 1 7/8 primary. Talk to Streetwize here on the board. A 3" collector will limit your torque peak on a big motor, which will kill upper end HP.