A 3.5 is a bit too much motor for a 2wd buggy, you'll have trouble keeping the front end on the floor. Chances are if you adjust the slipper to compensate you'll cook the pads and plates, leave the slipper tight and the poor diff is going to have a very hard time
At a guess start in the 17-18t range of pinions and see, though being a 3.5 it's going to get red hot no matter how you gear it I'm afraid!