Ummmmmmmm
No, he can just buy the sure grip differential and ring and pinion. Housing is pretty much the same, he just has to make sure he gets the right carrier bearings.
Mopar 8 3/4" (8.75) Clutch Type Sure-Grip ("Powr-lok")
Mopar 8 3/4" (8.75) bearing rebuild kit
Mopar 8 3/4" (8.75) Ring and Pinion for 741, 742, or 489 case
It's also worth noting that '74 Duster's didn't come with 8 3/4 rear axles, so, you may want to do a little more research to find out what exactly it is that you have there. Based on your other thread, you have an open diff right now so if you want to drive both wheels at the same time you'd need to add the sure grip. And, before you buy new rims as per a few of your other threads as well, you'll want to measure that rear axle. Because it may not be an A-body width 8 3/4.
As far as gear choice, 3.91's will be a lot of fun off the line, and absolutely miserable on the freeway for more than 20 minutes. Not sure what your planned use for the car is, but if it includes driving at 65mph or more for an hour or more at a time you may want to reconsider 3.91's unless you're installing an overdrive as well.
