I am about to drop a cummins 4bt in my 53 1 ton. I chose it cause the power band is a little wider than the 4-53. In stock form 120 hp at 2500 rpm and 327 ft-lbs at 1700 rpm. You can pump them up if you are so inclined. If it goes as expected, the 39 1.5 ton won't be too far behind.
Went to moultrie with a guy in a 2002 dodge dually with a cummins and a 5 speed hauling a goose neck with a nova and a 52 sport coupe. Averaged 15 mpg at 70 mph. Unladen he gets 21 mpg!
the 6 cylinder cummins is every bit of 39 inches long, maybe a gmc could swallow it, but not my chevys without some hacking.
