318 poly in 68 cuda

The tranny came with it and was bolted together. I took one of the motor mounts off the motor and it doesnt line up with the mounts on the frame. i bought the cuda without an engine. I bought the engine on ebay and thought it was an LA. Im new to the muscle car work, I can tell you every thing about a an early jeep. The A engines are about 80lbs heaver than LA blocks. I was wondering about how long the drive shaft would be.