The axles have to match the housing ends.  8.8" (Fox) axles were c-clip axles.  The bearings were pressed into the tubes in the housing, and the axles retained by c-clips inside the differential.  9" rears have the bearings pressed onto the axle shaft and are retained by a four bolt retainer that sandwiched the drum brake backing plate to the housing end.  The axles need a pressed on bearing and a retainer I would suppose, but I would ask whoever welded on the housing ends.  Hopefully this was a kit or something and someone isn't playing cut and paste with random parts.  If that's the case you may need custom axles. 

Out of curiosity, why would you want to use the Fox 4-link?  It was a terrible setup.  Basically Ford got cheap with a 4-link and decided that if they angled the control arms they didn't need a panhard rod.  Problem is that the control arms don't move in the same arc during compression, and at some point they just bind.  This will happen in the middle of a hard corner and the rear tires will immediately break loose and put the car into a spin.  I've experienced this first hand and its unnerving the say the least.  I would look to either put a true 4-link in the car with a panhard rod, or look at a torque arm system.  The 4-link is good for drag racing, but the torque arm is by far a better setup for handling.  If the subframe will accept Fox parts possibly you can just go with Maximum Motorsports torque arm kit and panhard rod kit.  I'm betting there'd still be some fabrication, but it would probably save a lot over just starting from scratch.  I'm working on an '89 GT project car now, and that's who I'm using for everything.  

Last edited by TKOPerformance (5/24/2017 6:15 PM)

As far as brake fitment I will defer to whatever Musang Steve says, but I can tell you that the 8.8 and 9" housing ends do not have the same bolt pattern, so if any stock kit will work I'm betting on the 9" kit.

Here's a link to Maximum Motorsports Rear Grip Package:

http://www.maximummotorsports.com/Torque-arm-Package-1979-98-Mustang-P341.aspx

This has everything in one kit: the lower control arms, the panhard rod, and the torque arm.  The factory lower control arms are junk.  They were so flex prone that Ford added quad shocks to dampen wheel hop on hard acceleration.  The Fox upper control arms are eliminated with the use of the torque arm system.  The panhard rod positively locates the axle housing side to side.  In this system each link does one thing, instead of each link trying to so multiple things as in the stock Fox setup.  Whenever something must do two things it tends to do neither very well, which is why the Fox design is so compromised.