I installed 10"x2.5" drums on the rear of the mustang 8-10 years ago. Ford offered a kit for a short period of time back then.I purchased everything together, backing plates to brake drums, and everything in between. I think the brakes came off a Ford Aerostar.
The mounting holes in the backing plates didn't match up to the housing perfectly, so I had to egg-out the backing plate holes a little to get them mounted, which wasn't a real problem. But, the centering holes in the brake drums are a little larger than the round end on the factory mustang axles. This allows the drum to be installed slightly off center... which can make the brakes feel warped.
It's kind of a pain, and I wouldn't fool with it again. Aftermarket liteweight rear disks are now becoming affordable at a few hundred dollars... and if you've got to have Ford parts, Motorsport offers a kit to install rear disks from an Explorer at a decent price.
Good Luck!
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Mike Burch
66 mustang real street
302 4-speed 289 heads
10.63 @ 129.3
http://www.geocities.com/motorcitymustang/cmml/cmml_mburch.html
http://www.fortunecity.com/silverstone/healey/367