My problem is that there is no good transmission to go behind the Cummins engine. I'm currently getting my NV4500 rebuilt for the second time in 277k miles. Maybe the fact that I'm putting 800+ ft*lbs through a transmission rated at 500 ft*lbs has something to do with it. I took my truck to my local trans shop complaining about hard shifting when cold, so they took it apart and found the clutch plate was worn out, the input shaft bearing was worn, shaft end play was excessive, the fifth gear pinion splines were worn, and 4th gear was missing a tooth. I looked into swapping an Allison 1000 into it, and after an internet search found that it would cost ~$12,000, plus some very careful programming of the TECU shift timing, because the Allison shifts clutch to clutch. OK, so I guess I'll live with spending a few thousand dollars every six years to rebuild the NV4500.