There are numerous sources...
There are numerous sources for T5s, which is the most popular choice for a five-speed swap into vintage Mustangs. A T5 can be obtained used, or freshened-up units are available from D&D Performance. A brand-new T5 "Z" trans shown here (PN M-7003-Z) is also available from Ford Racing Performance Parts or any dealer that stocks FRPP components. If you're thinking about a used T5 out of a Fox 5.0 donor car, stick with the '90-and-newer versions as they are the strongest and have the most upgrades from the factory. The T5s found in '94-'95 5.0s won't work because those units have a longer input shaft which won't work in a vintage swap application.
Another route is a used AOD from a donor car. The best choice is one from an automatic-equipped '86-'93 5.0 Mustang, all of which were equipped with AODs. For a carbureted vintage Mustang, the AOD-Es and 4R70-Ws found in later 5.0s and 4.6s aren't good choices as they are computer-controlled and can't be set up as easily in a vintage car. Stick with a Fox-body AOD from a '93 or older 5.0, and you'll be in good shape. The torque converter in a Fox 5.0 will also work in a typical vintage Mustang.
The next main component you'll need is an aftermarket crossmember because the stock one won't work. Several sources offer automatic- and manual-transmission crossmembers for '65-'73 Mustangs, including Ron Morris Performance, California Pony Cars, DB Performance Engineering, National Parts Depot, Mustangs Plus, and CJ Pony Parts. One stock piece that will work is your car's existing C4 transmission mount. It's just the right thickness to fit between an AOD and a crossmember designed for AOD swaps into vintage Mustangs.
Moving on to the smaller but equally important pieces to complete a conversion, you'll also need a throttle-valve (TV) cable that does the equivalent for an AOD that a vacuum modulator and kick-down linkage does with a C4, FMX, or C6: It makes the transmission shift properly. With a carbureted engine, both Lokar (PN KD-2AODHT) and Ron Morris Performance offer TV cable setups that will work with an AOD.
Wrapping up the laundry list of required parts for the typical vintage-Mustang AOD swap, you'll also need an AOD dipstick and tube (the ones from a Fox 5.0 donor car will work). A shift linkage that connects the AOD trans to a car's stock shifter (which will also work) and a flexplate are both available from Ron Morris Performance. An AOD yoke can be ordered from a Ford dealer. You'll also likely need to relocate the transmission lines where they connected to the C4 from the radiator.
Ron Morris Performance offers...
Ron Morris Performance offers an AOD crossmember for '65-'66 Mustangs. It features TIG-welded construction, a black powdercoat finish, and comes with new mounting hardware. The lightweight unit allows easy access to all four transmission-mount bolts.
In addition to an AOD yoke to be installed on the driveshaft, the car's driveshaft needs to be shortened, around 1 inch in most cases. Each car should be individually measured to confirm.
Parts Needed for an AOD ConversionAOD transmissionAOD crossmemberC4 transmission mountFlexplateThrottle-valve (TV) cableAOD dipstick and tubeShift linkage to connect transmission to shifterAOD yoke
Any vintage Mustang equipped with a three-speed manual, T-10 four-speed, or Top Loader four-speed is the ideal candidate for a T5 five-speed swap. Several other five-speeds are available these days, including the newer Tremec TKO-500 and TKO-600 five-speeds, as well as the burly T56 six-speed, all of which are available from Keisler Engineering or D&D Performance. While these three transmissions are pretty much bulletproof and installation is similar to a T5, we're going to focus mainly on the T5 conversion as it's the ideal five-speed overdrive transmission for any stock vintage Mustang with a 289, 302, or 351 engine.
For the typical AOD swap,...
For the typical AOD swap, Ron Morris Performance carries a flexplate that will cover the majority of applications using a 289, early 302, or 351W engine. It has a 28-ounce/inch unbalance and a 164-tooth ring gear.
There are similarities with T5 and AOD swaps; the most apparent is that you'll need an aftermarket crossmember. As with AODs, numerous sources offer T5 crossmembers, including Ron Morris Performance, California Pony Cars, DB Performance Engineering, National Parts Depot, Mustangs Plus, and CJ Pony Parts. There are two ways to do a T5 swap, depending on what you want and/or whether your car is a manual or automatic to start with. The next section is more applicable to cars with automatics, so here we'll assume an existing vintage Mustang is factory-equipped with a three- or four-speed manual trans.