My B1500 2000 Ram Van has a 3-speed with no OD. So, I do know that the 3-speed was still offered then as well.
As for putting a tranny from 1998+ newer into a 1997, take into consideration that the engine and tranny for the 1998-2003 were moved forward 4-inches to allow more front driver and passenger room and to allow the fuel tank relocation to the mid center of the vehicle. Therefore, the drive shaft or the tranny may need to be lengthened or shortened depending on how you look at it.
As for fuel economy, I have a 2001 B2500 with the heavy HD suspension, 8-lug 16 inch wheels, seating for 12-passengers, AND the 3.92 rear. With all of that said, I'm now getting 16Hwy as of my last two fill ups.
Much of this mileage increase was due to a few things. Running with Autolite AP5224 plugs, copper/brass cap-n-rotor, higher flow exhaust, high flow CAT, new O2 sensors, 5W30, larger oil filter, synthetic rear axle 75W90 oil, Teflon (PTFE) synthetic wheel bearing grease (over pack the snot for the front inner/outer bearings is a must), and new self rear adjusters for the back brakes to reduce further drag.) Plus, I do about 60 on the interstate vs. the 75 I once did. So, I leave 10mins earlier, but the fuel savings is a win.
When my radiator goes, could be 6mn to 1 yr, I plan on the eFan then. My biggest problem is to curb the small rust issue right now.