If you go for a 91 then you really don`t need to use a big pipe because you already have more than enough power, just bolt the engine on and go.
 You can use a 12x10 and will not have ground clearance problems.

The Tower 75 I had turned the APC 12.25x3.75 at 15000 rpm even (I did mod it a little) But an APC 13-6 turning almost 13000 rpm on a 91 is even better and gives better vertical and also more speed.