Transmission Leak

So I don't know if this is the right place for this question, if it is not I apologize. I along with a friend put a stock TH 350 Transmission in a car, nothing fancy just a 68 Camaro with virtually nothing done to the motor. The transmission is fresh and never had fluid in it, the guy who sold it to my friend said it was new, seemed like an honest guy who even sold the trans at his house, so he obviously was not worried about it coming back on him as junk. The issue is the car moves fine in 1,2,3 and reverse, the problem is when the car is put into reverse fluid pours out of it like crazy. It is leaking so bad that it leaves a 6 inch spray pattern on the ground. I think it is coming out of the vent tube but it is so hard to tell. Any help on this is appreciated. I am at a loss.