The spacing on the flywheel is different when attaching a 4l80e to a few different things. The early 6.o only came with a 4l80e and had the spacer built in, and its difficult to retro-convert to use anything besides the 80e. The smaller cube truck engines that came with the 60e's can run a small spacer or switch out to the lq4 style flywheel.
The 80e uses more bolts on the tq converter, which seems so much more robust than the ls-car style converters.