1 Problem with my 69 cuda.

I am a mopar newbie. I just got this 1969 Barracuda Convertible, LOVE IT! The engine and transmission have been rebuilt prior to my owning the car. The car had set for some time before I got it. My problem is that it leaks transmission fluid. I thought the transmission pan gasket was old and leaking so I changed it. The car still leaks. It will leak just sitting in the garage overnight. What else would cause this. Thanks.