Too tall 7" without and padding, real close though. I ship everything that will fit just about in those boxes or its smaller brother. Not new to the shipping game as I have shipped hundreds of items. Used to ship UPS mainly but their prices went up so much I mainly use USPS now, would ship more Greyhound if it wasnt for the distance to one for me and some of the seedy locations that you have to pick items up at. Appreciate the thought though.
I know it may be a p.i.a. for him but I would take the tranny and converter to builder or someone else if builder is far away and ask what is up. Too much beating your head against the wall for what should be an easy task. At this point it has to be tranny was put together wrong or converter is wrong or not getting it seated all the way due to an unknown factor. If it wasnt for expense I would have the other transmission rebuilt and use it due to the fact of beefier input shaft and valve body differences.
For sure I would also be back to the guy that rebuilt it, you paid him to do it so he should be the one to fix the screw up.