Ok the rear end on my 73 Charger restoration is the only questionable system that wasn't totally gone through. It's a 8.75 I think 741 case, 3.91 gears and has limited slip. The only thing I did was take the case off and had an older mechanic look it over. He said all the bearings and everything inside looked fairly new. He just put a new pinion seal on and that's it. I put of course a new carrier gasket, and I also added new wheel seals, along with the additive and new fluid. When I got everything in I hit the one side axle and then went and hit the other side to check the endplay. There really wasn't any play. So when I take out the Charger for the initial drive I'm pretty confident about everything else on the car but not sure what to expect from the rear end. Are they pretty reliable and forgiving with the specs if they're not dead on?

You need some endplay. Get it up on stands.
Loosen the adjuster until you have some endplay.
Then tighten it until you have none. Then back it off about 2 notches.

As for how forgiving they are. What part of a car that's almost 40 years old can be trusted?
I have 2. One is in a 65 Belvedere that leads a very easy life and it's just a nice weather driver so it's never been opened and I trust it 100%. My other is in my truck. I drive it everyday, it hauls, it tows and it's part play toy. So this one got everything. All new bearings, seals and gears.

The 8 3/4 axle is a good unit. The limited slip unit is designed so that even when it wears out, it will still operate as if it is a conventional open diff. The majority of our classics came without a limited slip differential, so when yours turns into the one wheel peel, it doesn't hurt anything but your timeslips!

If the unit the clutch type sure grip you can put in new clutches. If it hasa the cone/Auburn type unit. When it wears out, drive it as an open unit or buy a new one. If you're not racing, a sure grip will cost you 1+ MPG.

The 8.75 is agood rear end often tossed for no more then racing babble that preaches if it can't handle 600 hp its no good. Keep the race parts on race cars but the majority of the cars are hanger queens period.Yes I know I beat the ever living crap out of my car and the 8.75 became nine pieces. I have seen every make explode break you name it but facts are 8.75 are a good rear. I have seen a Dana 60 break at the pinion 9 inch ford the ring gear and 12 bolt chevy well never mind you get the picture!