actually in all honesty, the 8inch is just as strong as the 9 inch and 100lbs lighter. Alot of the internals in the 8 inch are the same peices used in the 9, like spider gears and side gears. Just check out this article at ford muscle http://126.96.36.199/archives/1999/12/lockright/index.shtml
I greatly appreciate the help, i will probably put a detroit locker into it because of how well known it is and it comes highly recommended.
I think the 9" is about 15 pounds heavier than the 8". The welded housing assembly is 5 pounds heavier, the center section 10 pounds heavier. If 31 spline axles are used add another 2 pounds. Max difference can be 17 pounds in that case.
I am not aware of any Detroit Lockers made to fit an 8" rear. Where did you find that information?
From reading the article, I believe they are implying the entire rear-end would be 100 pounds lighter. However, I don't see how this could be. I'm with Royce, the center section probably isn't more than 15 or so pounds lighter. As for the housing itself, have you ever lifted a 8 or 9 inch housing, without axles or pumpkin installed? There is nothing to it. And, the axles are common between both setups. 100 lbs, I don't think so...
You have to swap housing and all.
My father has an AUburn posi in his 8" 66 Mustang and loves it. It is a clutch type. It's doubtful that you would need a 9" with a small block. I would swap to 9" only if you had a complete one at a reasonable price.
Use the link in my earlier post to go to Drive Train Specialists. They list the Auburn for the 8". Around 430 I think. Summit or Jegs may have them as well - I believe I have seen them in Jegs catalogs.
Just to give you an idea, last year, I spent a total of $490 on parts and labor, to have my 9 inch center section built from a 3.00 open to 3.25 trak-lok. That was with me removing and re-installing the pumpkin. 9inch housing aren't very expensive, less than $100 for a good used one.
8" was used in 6 cyl and small block Mustangs and Mustang II through 74 I believe. How many of them were locking? Probably none. I don't think Ford offered a locking 8" from the factory. Locking 9" is no easy find now a days either. For a 9" I would look in pickup trucks.
The 8" rear was used at least until 1978 in the Mustang II, Granada, Mercury Monarch, Maveric and Comet. Posi's were a factory option from 1963 until 1978 in 8 inch rears but they were extremely rare. I have seen a lot of rare and unusual things in all the Cougars, Mustangs, Falcons and Comets I have owned but none had a posi.