There are several that you can buy that will bolt up, but the most common, and cheapest, will be the T5.
It came in several different vehicles, but you want to get one specifically out of a 5.0 Mustang, '87-'93 range. The T5 has a cable operated clutch (assuming you use the stock Mustang bell housing), and will require you to modify your car accordingly. The easiest way is to use the stock Maverick clutch pedal, and get the clutch quadrant and cable guide from Mustang Steve. Earl Branham on this site has put a T5 in his '69.5, and would probably be happy to tell you exactly what he did.
i agree with Jamie. the 4 cylinder t-bird and mustang T5 trannies will bolt in, but arent capable of holding up to a V8. the 3.8 T5 from the 94-98 mustangs will work if you change the input change and bearing retainer, as will the 94-95 V8 T5 with the same mods. needless to say your best bet is fox-body V8 T5 from 87-93, which are easy to find for $300-$400 all day long and on a personal note, i plan to run a T5 in my '73 and im going to use the cable clutch setup like a mustang with the quadrant and firewall adjuster
I was going to say the same thing. Many 5 speeds will fit the car but only fit the engine with the correct bell housing. The Tremec 5 speeds will work with the 79-83 SROD bell, or an earlier Toploader bell with a 1" spacer between the bell and transmission.