Royce, agreed on vent tube. The dipstick tube is still in place. It is a Lokar and is a perfect fit with double orings. After some thought, I dont see how the trans would still operate if the case cracked because it seems it would have all kinds of internal pressure leaks. I did pull the truck on the trailer under its own power. The leak is definately not related to pressure as well. We jacked up the passenger side and it quit leaking. Let it down level and it pours out untill oil level goes down.
It is funny you have had better luck with TH350's. I went through a couple TH400 rebuilds and switched to a TH350 on my 57 with no problems. Of course I think that was a pump failure problem from increased pressure. I am not using a brake so I am hoping the TH350 will live until I go with a Powerglide. The friend of mine who built this transmission has run TH350's behind 700+HP big blocks. Of course, like you say, it takes a set of clutches once in a while. High gear being the main problem from my understanding.