Rear diff
 

My 1968 Newport has an 8-3/4 rear end in it. I'm trying to order a sure grip for it with 3.55 gears. There is no tag on the diff, and the car sits underneathe 2 feet of snow, so trying to check a number on the case is near impossible. Actually, it is impossible right now. Not sure how many splines on the axles. I am pretty confident it's a 742 case, as I believe the 489 came out after 1968. The 741 case was more in A and B bodies from what I've read. The car came with a 383, and 3 speed 727 from factory.

Sounds like your still going to have to pull one of the axles to make sure what you have for the axles. The case has the casting on the side....
Cant help with getting rid of all that snow....
We keep our Snow up on the mountain...
Got to Love SO-CAL in that respect... :)

Bob nailed it.
Any of the 8 3/4 center sections will interchange (except the really early units), the difference is the pinion stem and axle bearings.
As far as your case #...I don't have a clue, even my 67 Sat (B body) factory BB, sure grip, 727, is a 742 case.
If you're just wanting to swap the third member and keep the same case #,
Unfortunately, you're going to have to physically look,
Drivers side, just behind the pinion yoke, last 3 numbers.
Here's an example, this is a 741 case.
Attachment 12388
I'd guess yours could be a 741, but possibly being a factory BB car a 742,
Typical rule of thought is in the 8 3/4 the 489 is strongest followed by the 742 then lastly the 741 since it has the smallest diameter pinion stem.

bob here in Kansas we keep our snow on the mountains to. mount sunflower 4,039. FT. LOL

Hey Moe,
Whereabouts in KS are you?
I'm in KS also, central KS.
We've been lucky so far this winter *knocks on wood*
Hopefully I didn't jinx us lol

iam in wichita. we have good mopar club here. i was one of first in the club in 1985. i droped out fof a while but iam back in now.i think we are going to have a swap meet in Jan or Feb. but it is only on a Fri and Sat, NOT Sun. i ll check it out and let you know. update: go to http://www.wichitaas.com/swapmeet/swapmeet.html
down load the swap meet flyer for info.

Thanks Moe!
Wichita is about an hour from me, so not far, I'm past emporia out by Ottawa.
I've been looking for swap meets in this area but can never seem to find any, so ill def be checking this one out!

Wind City Kansas..... :)

Ok, so by pinion you mean the splines on the axle?
Also, just to verify I have this correct, if I find a case with a posi unit in it, and the unit has the same number of splines for the axle, then I can just swap it out?

I think the OEM axles have a common 30 splines.. for all A B C trucks... ANY 8 3/4
The optional 35 spline axles are aftermarket racing
8 1/4 has 27 splines 9 1/4 has 31

As usual TVLynn is correct,
Are you wanting a limited slip (sure grip) or a "posi" (spool/welded)? There is a difference.
Your axle splines are irrelevant (basically...) for a standard 3rd member swap.
When I say "pinion" I'm referring to the gears, the pinion gear that's in contact with your ring gear and the pinion shaft that your yoke and driveshaft mount to.
The 3 case #'s (489, 742, 741) differences are; axle bearings, pinion stem diameter (741=1 3/8", 742=1 3/4", 489=1 7/8")
Yes, all 3 cases will interchange within the same housing, AS LONG AS, you use the correct axle bearings for the application (avoid pre 1964 center chunks though, something about a thrust block difference, I don't recall exactly at this point...)
So, if you can get a good deal on a "known" good, third member, its a straight swap (once you change to the correct axle bearings per case #, its a good idea anyway to change them out while you're there...)
BUT, just an FYI, if its a sure grip unit you're after (which is a good idea for a street driven car IMO)
There are 2 styles of sure grips; a cone sure grip unit and a clutch sure grip unit, either will work for you, but the clutch is preferable IMHO.
Attachment 12385

Perfect. Now I am clear about how these rear ends work. I don't know why, but suspension, and differentials have always scared me. Thanks guys!

posi is a gm term for a sure grip

to zombie check this out. http://www.wichitaas.com/swapmeet/swapmeet.html
on feb 5-6 i think. ONLY on friday and satrday. NOT sunday

For the 741,742,489 Can use the same axles. The early clutch sure grip needs special attention for the end buttons for axle end play

My 64 741 uses buttons while the 489 3.91 I just put in does not.

The 741 is a 2.76 sure grip by the way.