Make sure the right convertor is being installed.
It's pretty much a "feel" thing sometimes. I can tell you that you need to hold the convertor level (not let it hang on the input shaft/stator support). While holding it as square as possible, spin it while gently pushing in on it. Try wobbling it around a little while doing so. It's hard to explain, that's why I say it's kind of a feel thing. Sometimes it's easier to stand the tranny on end, but that's usually when you have a new bushing that is a little too tight or has a ding from installing.