Rear Diff Decisions

[boxfox]

Alright so I'm trying to figure out what I'm doing or what I need to be doing, I'm looking at a 3.23 sure grip rear differential with a 742 case, that will be replacing what I have currently which is what I assume to be a 2.76 nonposi with a 489 case.

Now the question is since supposedly the 489 is the more heavy duty, would I want to keep that/can I just put new gears into the 489 case and a sure grip unit..
I have never gotten the chance to dig into a rear differential yet so I'm pretty virgin on what is what with it.

The car is a 72 Challenger, 440 engine, 727 3 speed transmission automatic.
I'm also trying to weigh 3.23 against 3.55 for my choice, I do a fair amount of highway traveling, have 235/60/14 tires on the rear currently, might put on some 265/60/15s for when I hit the drag strip. or if I decide to run bigger tires how would that effect the rear diff.

Thanks in advance guys, I'd be lost without you.

[boxfox]

Yeah I think the 3.23 will do me just fine, I'm more wondering if the 489 case is worth using again or if a 742 will do just as fine

[Moparal]

you wont break a 742 or 489  with a 323 and an automatic.  Not with street tires. If the 742 is at a good price......buy it. It should be cheaper than building one from scratch

[Moparal]

a 742 , if it's a powerlock, will cost 60 bucks or so to rebuild.   To build one  with all new parts and you do the work yourself can be around 800 unless you find some fantastic deals on the parts. But bearings, seals, shims, gears and sure grip will add up in the cost.  Used parts dont have to meen worn out or cheap. Bargains are out there. It's up to you want to do.  again, it is doubtful that you will break the 742. 

[Chryco Psycho]

I prefer the 742 I feel it is the strongest carrier & usually has the clutch type sure grip which is also superior , the 489 has a crush sleeve which is the weak link , if you use the 489 I would replace the crush sleeve with a spacer & shims so it cannot fail ,

[inviolet]

I'm going through the same decision now too. It's either a 742 with 3.23 or a 489 with 3.55's. However I'm looking to do a Detroit Truetrac  instead of the standard clutch type.

http://www.eaton.com/EatonCom/ProductsServices/PerformanceProducts/Products/Differentials/DetroitTruetrac/

Currently I run a Tremec TKO500 5-speed with the 3.23's in a 489 case , and I love the highway ride but I could use a little more pep in 4th.

Gear Calculator and Tire Calculator below:

http://www.americantorque.com/graph-rpm-vs-speed/

and

http://www.rimsntires.com/rt_specs.jsp

[boxfox]

Alright so if I go with 3.23's can I still get my Challenger to see the low 13's and maybe even better than that? I'm not going to make it a 10 second car until maybe way later on in life but in the mean time I want to be at least a mid 13 and I'd assume I'm a mid to high 14 right now. What do I need to do to shave off another second? What are my options? I'd rather not cut a hole in the hood for a supercharger.