Mack transmissions are bulletproof; even the 5-spd designed for maxidyne engines will easily stand 1600 ft-lb torque. I'd be concerned about the brownie, too, though, until you find out exactly what you have. I don't think the max. torque figure is achieved at 1250 on a K. It has been a long time, but I'm thinking 1500 or 1600. And they will burn if lugged, so you need to find out about that. A 1600-series 18-speed fuller would be an excellent choice, as would the 16713B, but the 13B I have has a shorter O/D.
