C4 to Toploader conversion...

Most any 289,302,351w/c bellhousing will work.
Some NOT to use: Early SBF with 5 bellhousing bolts, late SBF with hydraulic or cable clutch.

Some differences among those that will work:
10.5" clutch bellhousing. 289/302 part. It is aluminum and uses a small flywheel/clutch.
11" clutch bellhousing. 302B, 351c, and truck part. It is cast iron, much stronger, but also heavier. Uses larger flywheel and 11" clutch. This is my preference and I have some laying around in Memphis. Worked well on a 302.

Wide ratio all the way for the street. You can run a milder gear while getting low gear performance.

You need to get the short Toploader. It is the same length as a C4 and uses the same driveshaft. It is the only length used in any Mustang. The longer tranny was used in Torinos and larger. It was made to interchange with the C6 and would be odd to try and put in a Stang.
The input shaft you need is the smaller of the 2 available.
Be careful, the small input/short tail toploader was not only used behind SBFs, but also FE engines. They look identical except one small, but damning, detail... The tip of the input shaft on the FE tranny has a nose that rides in the crank bushing that is about 7/8" long. The one you want has a nose that is over an inch long, 1 7/16" if memory serves. Don't worry, there are only 2, so if your's is not under an inch, you are okay.
The toploader uses a different starter than the autos.
For pedals, get the whole assembly out from over the steering column from a donor car.
The speedo gear works from a C4, but if memory serves, the cable does not reach well.
Good luck
Dave