I doubt that your going to find a 6 speed tranny made for your truck, I mean you could probably find one from a cobra mustang and have it mechined to fit your application, but the gear ratios aren't made for truck applications. If you have a stock 351 in your truck then its a modified, or a windser, I belive eighter way you could transplant the cobra tranny in there, but you would have to have a new shaft, or have the original one mechined, and I would talk to someone at a tranny shop to see what gearset would be best, plus you would have to find a kit for mouting it to your crossmember, and have your driveshaft shortend, but if you have a 4x4, thats a new can of worms
the differance between a 351 m and a 351 C is that the 351 C was a small bore/long stroke motor to be more big blockesh, and the 351 M is the best of both worlds, it was sappose to take the good aspects of the 351 C and combine them with the 351 windser, thats why allot of parts from the 351 C are interchangable with the 351 C and 400 M, hope this is helpfull
