Joined: Aug 2002
They made quite a few small bearing 9" rears, and that's what's correct for a Mustang and most 'car' rears, not to mention that's what most of the rear disc kits require.
But, it seems like everything I've seen in the junkyards is in a later model full size car or a truck, and 99.9% of those used large bearings, so it's definately an oddball to find a small bearing rear lying around these days. I've found plenty if I look for one specifically for a mustang, but I've saw ONE in a junkyard and the guy was busy getting it ready for his 67 Coog.
Here's another odd choice, a 94-98 Mustang GT 8.8". They are posi, disc brake and exactly the right width for a 71-73 Mustang, just add spring mounts and torch off the sway bar and coil spring junk. Really cheap too, since the weird axles make the assembly too wide to fit other cars.
Your best bet though, since I see you said you already have a 9" is to do the SVO mustang or Crown Victoria disc swap and put in a posi 3rd member. Crown Vic swap is like $150 and I've got less than $325 in my SVO swap...the 3rd member, assembled and ready to bolt in is probably $600.
66 coupe 5.0/T5, home-brewed power 4 wheel disc setup, 350hp+, looks like it belongs in 1968