From what I've read, those who have done the 4BT conversion wished they had just gone ahead and went with the 6BT when all was said and done. The major complaint with the 4BT is major vibration especially at idle and noise. Being that it's a 4 cylinder, it's unbalanced while the 6 cylinder is balanced so it runs a lot smoother. Both engines get good gas mileage. My Cummins Suburban gets 26 mpg on the freeway and I have it geared such that it turns just a tad bit over 1300 rpms at 65 mph. These engines like to cruise at RPM's under 1800 rpm's. If you're going with an OD transmission, you want 3.43 or 3.54 gears and without OD, you want either 3.08 or 2.73 gears.
The 4BT or 6BT should be able to be shoe horned in there without any body lift. My 1990 Suburban didn't need any body lift even with the huge 6 speed NV5600 transmission behind the Cummins.
The 4BT can make major power even if it has the rotary pump. The rotary pump is easy to turn up. On a 6BT, the rotaty pump is good for 500-600 hp while the P pump is good for over 1000 hp.
Is the 1972 Dodge 2 wheel drive and 3/4 ton?
1990 ¾ ton 4x4 Chevy Suburban
-Cummins Diesel - 12 valve - factory rebuilt
-6 speed bullet proof manual transmission - NV5600
-Gear Vendors Overdrive
-Upgraded Holset HX-35 turbo
-NP205 iron transfer case
-3.73 gears
1982 ½ ton Chevy Suburban
-6.2L diesel - high nickle crack free 1982 block
-Stans headers
-Ported heads
-Timing gear
-4 speed automatic
-3.08 gears
-30 mpg on freeway