3 speed to 4 speed conversion

A "Mustang" small block transmission (wide or close ratio) will fit in place of the 3 speed. Same length. You would also need a shifter and (I believe) the tranny yoke from a toploader or a C4. Easy swap with the right parts.

The Rug-C is NOT a good candidate as it was originally spec'd for a 390 and would have the wrong input shaft length and the shifter mounting is in the wrong location. Look for a tranny from a Mustang (65-73) or Cougar (67-73) with a 289, 302 or 351 and you would be good to go.

Check out this site to learn more about toploaders and how to ID their parts: http://www.4speedtoploaders.com/

He needs a Mustang, Fairlane, Cougar or Comet small block Toploader. For it to be a bolt in swap. Wide ratio is prefered. And I ditto Dennis's findings on the 390 transmission, unless you can get hold of the thicker Boss 302 pilot bushing, also used on some early Bronco's too.