Hy bigwolf, that sounds like a very substantial load, so I think that would rule out something like the NV4500. The Spicer, Clark, and New Process transmissions from that period are probably up to the task, and they can be had in both direct and overdrive fifth gear models. There are also newer transmissions available, some six speeds I have heard about, you should inquire at a truck wrecker or truck transmission shop, and see if they have any recomendations, hope that helps.
