I recently put a TH 400 transmission in my '71 T/A. Before I put the car back on the ground I put the transmission in drive while still on jack stands to make sure that the tires would turn. They did so I excitedly got the car back on the ground and prepared for a nice long burnout. I put the car in drive but did not feel the transmission engage. I them attempted to take off but the engine just revved. I checked the fluid level and it was fine. I again attempted to move the car but it wouldn't even try to move until the engine hit around 5300 rpm. Even then it barely moved at all. I have a 3,000 stall and a B&M shift kit. I followed all the instructions while installing these items. The transmission was in a running car before but sat for about 2 years before I put it in. Are there any particular items I need to replace or should I just spring for an entire rebuild?