While the 6 B series will make some decent hp with little effort, it is a very poor fit in a Jeep and way too heavy to be practical. If you are still set on the 6 cylinder, I know where there is a 5.9 ISB but it doesn't have a VP on it and I would imagine you would be looking for a 12V BTA anyway, right?
If you are looking for a used 4 BTA (3.9), they came in all sorts of applications from various tractors to stationary generators to Frito Lay trucks. If you are looking for a new 3.3 CTD, I would suggest that you call Cummins East in Mississauga for a dealer that sells industrial 4 cyl engines.
That said, I hope you don't care if your Jeep is a touch on the slow side.....because it certainly won't be very quick with either the 3.9 or 3.3.
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03 3500 QC DRW 4x4 6spd- PRXB*MBRP 304 Duals*Commander/X2 Race*Icebox*homebrewed air horn
98.5 3500 QC 4x4 24v 5spd-bombed, 400k, broken
94 2500 4x4-SOLD-bombed, missed