American Powertrain

They have what you are looking for. 5 or 6 Speed, built to handle what ever and they have a 24 hr tech line. Could give them a call and tell them your specs and what you want and they will set you up.

My brother had a T56, 6 Speed with two OD's behind his built 350 bored .060 over and it has held up very well. Only issue he ever had turned out to be the Centerforce clutch. He had towed many a vehicle on his trailer without issues with it.

58' 3200 235 3-speed/OD