C-4 Transmission Problems

[Smitty]

Okay Ford guys or any Transmission gurus; I have a C-4 behind my SS Mustang and I tried it this weekend for the first time with the SS engine and the car was real sluggish. When I got to the et shack I bumped the car up to 1st gear and tires tried to grab. I bumped it back to high gear I got back to the trailer. After checking fluid I managaed to get it up on trailer and checked my linakge to ensure it was catching on the blanket. I found it was overfilled slightly and had pushed out the vent. After letting the car cool off I backed it off the trailer and proceeded to move the car in first gear, car acted as if something was holding it, I bumped second gear and the wheel wanted to drag again, almost like it was trying to go into reverse.
Does anyone have any clue what might be the problem, this is the first time it has happened. The trans has a reverse manual valve body with brake, I beleive it is a Dynamic. Any tips would be appreciated.