I will just state this because I have never had one apart myself (only saw my dad's apart at the dealer, how I know what the guts look like).
The 5th gear is external to the main trans body... it is in the tailshaft assembly... it doesn't require the trans to be removed.
It does require a special tool to tighten the nut that holds the gear on the shaft... but I think guys get around this somehow...
I believe the TSB on it only states about 3 hours to do the entire thing... make sure you have the TSB before you go to a dealer as the one who repaired my dad's 4500 didn't install the updated nut the first time through (didn't know it existed)...
You probably wouldn't need the new gear the TSB calls for (the original one looked perfect, but they replaced it anyway)... probably just need the new nut.
Any thing beyond that and I can't help.
steved