As far as I know, the sheave angles on the 108 doesnt allow a full shift if using a 1-1/4'' belt. I would pop the cover plate off, remove the spring, then slide the movable sheave tight against the stationary sheave. Now throw your existing belt on it and see how far down the belt sits while holding the clutch sheaves together. Chances are the 1-1/4" belt wont come all the way to the top. Do you happen to have a black streak on the clutch sheaves near the top???
Your engagement sounds perfect, but your peak RPM is too low. I believe you have PSI twins on your sled?? In that case you should shoot for 77-7800 peak if you have the pipes that have the rings on the pipes that seal the flanges. They had other pipes that ran a little lower, in the 7500 rpm range that do not have the rings on the pipes.