Ok, since I'm getting a spankin' new engine and a super rebuilt Muncie M20 - and since it's been confirmed the transmission has seen abuse (guessing that abuse helped the engine die too), I have about zero confidence in the differential. My car is a 71, but it has an 80-82 rear end. I have no problem keeping that the same, but I think I really need to pop that cover to see what the inside is looking like. But I'm beginning to think that I should go ahead and pull it and do a rebuild, and I'd use that opportunity to change the gears. Right now it's a 3.08. I'm thinkng of going to a 3.55 to get some more punch off the line. I'm not looking to shred tires and I'm also not concerned about high speed driving (70 mph +) since that will happen very little. I know it comes down to a preference, but I'd welcome some feedback and opinions. Btw, the engine will be a 375HP 350.

On a related note, if I don't do anything toi the differential, and it fails, can it freeze up and take a transmission with it? Or would it just rip it's insides out?

3.55 would be a good choice.
If the diff takes a dump, it will not hert the tranz.
Diffs usually give you a fair warning before they crap out.
Whether or not you decide to pull it out, you should at least change the fluid, and get some new lube in it. I sure hope it ain't gots the same junk the tranz did!

Yeah, I hope it doesn't have any junk in the trunk...

I'll pull the cover and take a look. Maybe I'll get lucky and it will be ok.

[QUOTE=jgoglick]

Ok, since I'm getting a spankin' new engine and a super rebuilt Muncie M20 - and since it's been confirmed the transmission has seen abuse (guessing that abuse helped the engine die too), I have about zero confidence in the differential. My car is a 71, but it has an 80-82 rear end. I have no problem keeping that the same, but I think I really need to pop that cover to see what the inside is looking like. But I'm beginning to think that I should go ahead and pull it and do a rebuild, and I'd use that opportunity to change the gears. Right now it's a 3.08. I'm thinkng of going to a 3.55 to get some more punch off the line. I'm not looking to shred tires and I'm also not concerned about high speed driving (70 mph +) since that will happen very little. I know it comes down to a preference, but I'd welcome some feedback and opinions. Btw, the engine will be a 375HP 350.

On a related note, if I don't do anything toi the differential, and it fails, can it freeze up and take a transmission with it? Or would it just rip it's insides out?

[/QUOTE]

I like to get close to a 10.0 "launch" or first gear ratio..

the m-20 has a 2.52 first if i remeber right...

3.55 = 9.0, 3.70/3.73 = 9.4, 3.90 = 9.8

in your situation..  i would probably go 3.70

 

If you do an overhaul on the rear or diff do your self and your diff a favor by getting Eatons new posi differential.It has the carbon fiber disks and the case is stronger than the stock unit.
(make sure it is for the 3 series gearing)
3.55 or 3.70 would be ok I have a 3.73 in my 76.
If I am not mistaken the 80-82 rear end is a bit weaker than the 68-76 so changing it back is up to you.
If you were looking for more strength and reliability check out Tomsdifferentials.com .He has a lot of heavy duty upgrades for the C-3 rear. Up to 1430 universals! I put on a one of his large carrier bearing caps. It strenghtens the rear conciderably. You can put on both left and right caps but Tom advised me if I were to do one it should be the left because the left side takes the most abuse. Other than ape s**t horsepower the right side should be just fine with the stock cap. Be advised you will have to drill out and tap the carrier bearing bolt holes to fit in larger bolts. If you are going to pull the rear then it makes it a lot easier to drill them out.