GM will be to about 2005 or so with a diesel. BB gas may have ended a year or two before that. I found a few 6sp manuals with diesels in 05 when I bought my dually, but went with an allison. Probably the best auto I have driven, and only one I got to last longer than half a clutch. BUT, still not as good in low as a manual in first. The allison is about an effective 5-1, vs a HD manual in the 5.5 or lower range. THe 4 sp autos are as high as 4-1.
Marty
92 Navistar dump truck, 7.3L 7 sp, 4.33 gears with a Detroit no spin
2014 Chevy 1500 Dual cab 4x4
92 Red-e-haul 12K equipment trailer