First thing I would do, is post this in the SC10 thread, as they will know a lot more than I do about this specific car...
But I will try and help as much as I can...
So after reading your post, it sounds like the tranny is free before you put the slipper clutch assembly on, correct???
If so I would imagine it's probably the bottom slipper plate rubbing against the tranny case...It seem like i've heard about other's having that problem a while back...Other than that go back and make sure you assembeled every thing correctly...There is a trick to the slipper pads...When you put the first one on[The one facing the tranny case]Hold the car to the side so gravity holds the pad down on the spur gear, then slip it up the shaft and hold it against the slipper plate...Once it's seated correctly turn the car back over and install the other...This has proven to be the easiset way for me...
Hopefully you figure out your problem, and sorry if I wasn't much help...
Good Luck, Racer53