C4 Back Pressure When In Park

[WTWaller]

Good afternoon, I am putting a 1965 mustang back together that I had recently purchased. I am having a problem with the tranny. I was told this was all rebuilt, but don't know for sure. I can start and run ( up on stands ), as long as the tranny isn't in park, it will do just fine. As soon as I put it in park tho, the fluid sprays out of the vent tube on the transmission.
Does any one know of this issue and can you PLEASE help me to figure why this is.

Thanks WT