On the loose bolt theories, I do know a guy who installed his 18 spline 833 with only one bolt holding it to the bellhousing. The others were stipped/didn't line up, so he figured he'd just let it fly.

The car in question is a plum crazy '70 440 Six Pack 4 speed Super Track Pack Charger R/T, mind you, with a white interior (pretty car).

Well he drove it around a few miles, proceeded to lay into it a bit, and around the time he tried to yank second he started to lose the shifter. Despite his best efforts to hold the tranny in the car with the pistol grip, he could not, and through the transmission tunnel it went!

It doesn't really make a point regarding your problem, but a funny story that relates somewhat, and might make you feel a little better with a laugh.