Please excuse my ignorance (paint & body guy) but I have a few questions about these 3rd gen factory rearends....
if a the rear has disc brakes is it a posi ?
there is one at the yard thats complete ,usually the guts have been removed on all the ones with disc brakes...
can a regular rear be converted to a posi ?
is there a posi with drums.
how can I tell if a rear is any good before I remove it from the junk car
How can I tell if a rear is a posi ?
I can get this rear, complete with brakes for 100.00 ...although I dont need one I do have a 54 P/U I can put it in...
at that price it seems like a good deal wether posi or not so if it isnt and I can update it later on I just might get it...
I put one in my 48 plymouth already that has drums and its been doing great.but I dont if its posi or not...
My 85 Iroc has disc brakes and I was told it was a posi but I cant tell the difference....

 

No, disc brakes does not mean it's a posi rear end. The only way to tell for sure is to remove the cover and look at the differential. I'm sure someone is going to tell you to turn one axle and see which way the other one turns, but this is very unreliable. Again, the only way to tell for sure is to remove the cover and look inside.

If the rear end has the old cast iron calipers then you are probably better off with drum brakes. The aluminum calipers are much better.

 

A junkyard posi may seem like a good deal but don't forget, these cars are getting old and a lot of mileage on them. Posi's don't last forever. They do wear out.

As mentioned above, the only proper way to find out if it has a posi is to remove the cover and look inside. You never know what a previous owner did during the life of the car. Even if it does have a posi, you'll want to change the oil anyway.

Since you plan on putting the diff under an old truck, it doesn't really matter. You can always install a posi at a later date however trying to improve a 7.5" diff isn't a good choice. A second gen 8.5" diff would be stronger or since you'll be converting to leaf springs anyway, go grab an 8.8" from a Ford.

Before you go randomly grabbing a diff from the junkyard, measure the width you'll need o see what fits best.

 

OK so I pull the cover off,what am I looking for inside there that will tell what it is?

 

There were only 2 limited slip (posi) differentials installed in the 3rd gen 10 bolt. Neither one is any good. The Auburn will have 5 coil springs in the center. This is a cone type differential and when it wears out it's scrap metal. There is no rebuilding it. The other is the Eaton Governor Lock. This is a lot less common in the cars, but very common in GM trucks. They have a bunch of small parts that tend to break, and sometimes the case splits in half and wipes out pretty much everything inside the rear end. I'd recommend passing on any of the 7.5 rear ends and look for something better. The FOX body Mustang GT 8.8 is a good one because they all had the Trac Lock differential, and they are 59 1/2 inches wide. The SN95 Mustang GT 8.8 is also a good one because they all had the Trac Lock and disc brakes with 5 lug axles.

 

Gottcha,so I'll just pass on it...I can get any rear in the yard for 100.00 so if it was you and you had your pick You'd use a ford because they're tuffer,right? so whats your first choice? I can fab and weld anything in I just dont know what...I dont want to rebuild it either I never did one. I'll just find one thats clean (no leaks) Right?

 

First you need to find out what width you need. If you need a 56 inch rear end and you buy a 60 inch rear end you are going to be in trouble. You also need to consider what bolt pattern is on the axles, and how strong the rear end needs to be. You can't just walk into a junk yard and point to a rear end and expect it to work the way you need it to. Gear ratio is also something to consider. You probably don't want a 2.41 gear or a 4.11 gear, so figure out what you need and then go looking.

 

Using that logic, find a 9" that's close to the proper width and drop it in. There's a lot more aftermarket support for the 9" than anything else. Buy a posi and put any gear ratio you want on it. One posi fits all unlike GM where they come in different series depending what gear ratio you want to use.

As for tuffer, a GM 12 bolt is a good option but finding a car 12 bolt is difficult. Car 12 bolts are better than truck 12 bolts. A Dodge 8.75 is also very popular. Finding a Dana 60 from a Mopar car may be impossible.

Being a leaf spring vehicle makes any diff swap easy. Cut the pads/shock mounts off a donor diff and weld them onto the swap diff. Swapping diffs though can cause a lot of other problems. Will you have a working park brake system. You'll probably need some sort of conversion u-joint. Unless you replace the axles with something from the aftermarket, you'll need rims with the proper bolt spacing for the diff you intend to use. Depending on the diff width, rims with proper backspacing may need to be used.

Depending how you want to drive it, the tiny 7.5" diff may be just fine but it will never be as tuff as other junkyard options.