If it was a "340/AT" and the ring gear with teeth came off of the converter for no apparent reason, then the fault probably lies with the converter manufacturer. Improper welding!
I lost a driveshaft at the 1/8 mile mark. Driveshaft broke in two, rear end yoke was broken, had an almost 1/2-basketball sized dent in the floor behind the passenger seat, transmission was literally shattered from the bellhousing to the tailshaft hanging together by the trans pan and bent converter still bolted to the bent flexplate. The converter bolts sawed through the cam plug area as well as the two lower wings in the back of the block where the bellhousing bolts to the block with one wing broken off of the block as well! The converter was bent and junk, but the ring gear stayed in place.