Just want to pass on a recent experience with a rear axle replacement. I looked through shop manuals and searched the internet for the problem I encountered and didn't find even a hint of information.
I ordered a replacement driver-side rear axle for my 8", 28-spline, '65 coupe. There is only one axle available (no variations) and it is advertised as "These are direct replacement axle shafts that require no cutting or modification to fit seamlessly into your Mustang." on the CJ Pony website. This turns out not to be exactly true. NOWHERE is it mentioned that there are multiple brake offset measurements that can only be determined by measuring the original axle. This includes the manufacturers' website.
I found this out $300 of labor and parts later. The replacement shaft had a lesser brake offset measurement than the original so the brake drum is solidly mated to the brake backing plate and brake shoes. With no gap it is totally locked up and will not turn. Not suspecting the axle we tried for some time to make it turn and identify the problem. We finally pulled the axle out and compared it to the original. There was about a 3/16" difference in the offset measurement. This explains what happened to the 1/8" that the original had. The original axle was put back in the car and I left with no gains and $300 in the hole.
I reported this on a Tuesday to CJ Pony and they said they weren't sure what I was talking about and had no idea of any problems. I emailed an explanation and photos that evening. I called for days trying to get an explanation and was always told they would look into it and return my call. After 7 days I was finally told that I had to deal with the manufacturer and they could offer no support for the problem.
I personally called Superior Axles while waiting for 2 days for a call from CJP and discovered they were fully aware of the problem. They even have a $17.25 spacer to fix the problem and there is even a good illustration of axle measurements on their website. They have a great tech support that offered the news about the differences. Too little, too late. I have requested that they share this information with consumers and vendors. If you measure the offset on your old axle and call them, they will even be sure that you request the correct spacer if needed. It's still an extra pull of the axle but less time spent with needless troubleshooting like we did.
The total absence of this information surprises me. I don't think anybody has shared this experience before online. It's a small percentage of axles but not the only one.

 

I finally had a chance to speak with someone at CJ Pony in a position to deal with this situation. We got it settled to my complete satisfaction. It's unfortunate that it took this thread to bring it to his attention. My motive was to educate others about this situation since I personally couldn't find it anywhere else on the web. This whole predicament is 100% communication failure.
I was just looking for a simple direct replacement and thought I had found it. I've learned my lesson. I'm about to start playing the lottery if Superior Axles' information is truly correct. All information available points to there being only one 8" axle available for a V-8. They claim about 5% had this unique variance that required a spacer. I definitely defy the odds by having 3 (of 3) of them. The mechanic pulled an axle out of a bad axle assembly he had laying behind his shop to get me on the road temporarily since it had all the parts to just put on and go. And it was a perfect match. The splines are more than doubled on the replacement to allow length adjustment, so why can't it be possible that the tooling on the flange end is also generic? It's easier to shim than cut. I would have gotten it right with just a few simple words in the description about possible variations. Commucation failure snowballed into more.
I've got a new axle with spacer, the original axle, and a borrowed axle in the car all ready to make another trip to the mechanic to straighten it out.