5 speed here with a grain of salt.
If you're buying used, then watch out for the early ones. Countershaft 5th and main drive gear are about a 1/4" narrower than the later ones. I think the service bulletin to change that was around '82 or '83?
I have had mine for 23 years in my '82 FXRS and have beat the ever loving dog shit out of it since day 1. In that time, 2 broken gears(the old thin main drive gear and countershaft 5th) bent one main shaft, broke 3 throwout arms, and cracked one door and one housing. I run a full set of the atlas springs, with an extra plate clutch, and an aluminum pressure plate.
I have upgraded to the Andrews gear set and shafts, sputhe billet door and have been trouble free since. I have also made my own throw out arm using a broken HD one. A bit of weld, a lot of filing to eliminate stress risers, and its rock solid now. Next upgrade for me is the carllini torque plate. I keep ripping the motor away from the inner primary. Did I mention I like wheelies?