9 inch 3.00 rear end

KBSTANG is correct, if it is of the common "small bearing" design, the 8-inch backing plates should work just fine. I'm thinking rebuild kits run about $125 from JEGs and Summit. If you take it to a shop to be rebuilt, count on spending about $500. I had a 9-incher custom built starting with the axle housings from my 66 8-inch grafted onto a nine-inch center housing. With 3.25 gears (new), 4-spider pinion set up and Trac-Loc: $1000

Glenn Morgan: 66 GT V-Burgundy Fastback 351w+toploader+9 in. TracLoc. Started out as a rusted-out Chicago-area crusher. After sacrificing a solid 66 coupe for its sheetmetal sub-assemblies, I have one solid (and expensive) work in progress!

That thing is a work of art. The local (Denver) differential guy sends bare 8-inch housings to some place in IL. Someone there cuts off the axle housings and welds them onto 9-inch centers. Since 65-66 9-inch housings are very rare and expensive, this is a very attractive alternative. All the local early Mustang/Falcon/Comet guys here swear by these things. He even added new wheel bearings, 67-70 brakes, and little gussets on the spring saddles to keep them from being ripped off the axle housings when you dump the clutch at 5k.$1000 is alot of money - but I think I've received alot of value here. The axle is probably overkill, but it is one thing I'll never have to worry about.

Glenn Morgan: 66 GT V-Burgundy Fastbac