Well, the 1966 Mustangs came with C4s behind both the six-cylinder and 289 engines, and since the 302 is identical externally to the 289, it will all bolt together - but there are a few gotchas to watch out for. First, if the Mustang was originally a six, you'll want a stronger C4 - the six-banger units aren't as strong as the V8 jobs. Most C4s of the early 1970s are stronger anyway, so you might want to just start with one of those and have it rebuilt with modern, good-quality parts.
Second, Ford changed the way 302s are balanced in 1981. If your 302 is a 1980 or older, no problem. If it's 1981 or newer, you'll need to change out the flywheel and harmonic balancer.
There are a few other things to watch out for, but they're off-topic here.