[big gear head]

Don't use an Auburn. They are junk. Get the Eaton Posi, Yukon Dura Grip or Power Trax Grip LS. Your car will have 26 spline axles and you will need a 3 series for the standard 3.73 gears.

Be sure to get a GOOD installation kit too. Tom's Differentials and Randy's Ring & Pinion have the best kits. Most others are junk. Ratech is the worst.

Tom's has US Gear, which is one of the best, and Randy's has Yukon gears, which are also good. Good quality gears will make your life much better. Cheap gears will cause you problems.

 

[big gear head]

Yes, or this http://www.summitracing.com/parts/etn-19663-010/overview/

 

[big gear head]

Summit sells the good Motive gears and the crap Motive gears. They also sell the good Richmond gears and the crap Richmond gears. The Summit gears are probably going to make noise. Get the good parts. If you want a cheaper installation kit then get the USA Standard kit. It has the good shims, but the cheaper Koyo bearings. Don't get any of the other cheap kits. Summit also sells Yukon kits, which are the same as the USA Standard, except they have Timken bearings.

 

[big gear head]

27 splines on the pinion shaft and 26 splines on the axles.

Yes, those gears would work.

Some people have got away with around 400 HP through the 7.5 rear end, but others have broke them with stock 305s, so you can't actually put a HP number on where it will break. The way it's set up, how it's used, what kind of tires are on it and a few other things can affect how it holds up. Any wheel hop can break it. If the gears and bearings are not set up correctly it will be weaker. Sticky tires will put more stress on it.

Here is something to consider. In '90 the 7.5 was upgraded with 28 spline axles. There are many good 28 spline differentials available for this rear end. This is a good upgrade for any 26 spline 7.5 rear end. There is a good chance that your axles are worn where the bearings run on them and they will need to be replaced. If this is the case then consider upgrading to 28 spline axles.

Another popular upgrade to the 7.5 is the T/A Performance aluminum cover/girdle. This cover braces the bearing caps and adds some strength to the rear end.

I'm not completely sold on the solid pinion spacer, but many people think that it is a must have for the 7.5 rear end. I have one on the shelf that has been sitting there for years and I haven't used it yet. I haven't had any problems so far with crush spacers.

 

[big gear head]

You probably already know that the G body 8.5 rear ends are rare and hard to find. They were only used in the Grand National, T Type and a few 442s. They are usually kind of expensive too. There are many after market bolt in rear ends like the Moser Engineering 12 bolt and a bunch of 9 inch Fords from many other companies. They are also going to be expensive, but much easier to find.

 

[big gear head]

Another option to the rear end is a FOX body Mustang 8.8. There are parts available to adapt the Mustang 8.8 into the G body cars. All of the Mustang 8.8 rear ends have the Trac Lock differential and there are a lot of parts available for them. You would need to convert it to 5 lug by having the axles redrilled or getting after market axles. You can also use axles from a Ford Ranger to convert them to 5 lug, but it will be the Ford 5 on 4 1/2 inch pattern. This is a cheaper way to go. I can't remember if it's the left or right side Ranger axle that fits the Mustang, but you can't use both Ranger axles because one is longer than the other.

 

[big gear head]

A used 26 spline unit is going to be an Auburn. I won't use a new Auburn and I sure wouldn't waste any time on a used one. If you find one that still works it won't work for long. When they wear out the are scrap metal.

 

[big gear head]

Everybody has different experiences. My experience with the Auburn is like this. I installed 2 new Auburns with new gears and bearings and both of them came back in less that 9 months completely worn out. I had to scoop powdered metal out of the bottom of the housings and axle tubes and replace all of the bearings in both rear ends. I have removed a few used Auburns and also had to scoop out the metal in the housing and replace all bearings. I replaced a complete 8.5 in a SS Impala that broke an Auburn and punched 3 holes in the housing. When I still had my rear end shop my suppliers wouldn't sell me Auburns because they didn't like them either, not that I really wanted them anyway. If a customer requested an Auburn my suppliers would sell me an Eaton for the same price as an Auburn just to keep me from using them.

My experience with the Auburn is that they wear out MUCH quicker than the Eaton Posi, they fill the rear end with powdered metal and they can not be rebuilt. They also chatter a lot more than the Eaton Posi. After going through a few of them with my customers I stopped selling and installing them. It just wasn't worth the trouble to mess with them anymore. It's well worth the extra money for the Eaton Posi.

I know that many people have used them for years with no problems, but I'm not one of those people. If you treat them gentle they will do pretty good, but if you do a lot of burnouts or abuse them they will not last long.