I haven't done it however years ago I researched the idea and found that the transmission craddle mount would need to be modified clear it on my 78 van (pickups have clearance) and gas at less than 1$/gallon would take a lot of miles to break even, of coarse that has changed.
Also the drive shaft length might need to be changed also and the torque is less efficient at lower engine rpms. (More slippage.) It might serve best to get a [email protected] traveler torque converter made for towing that has a lower stall rpm (less slippage) along with lower axle gears. (Changing from 3.55 to 2.94 increased mpg's on my van by ~2 withe the odometer/speedo corrected by changing to the apropriate speedo gear.)
Maybe I might just do a gear change. I know putting the van on a diet is out of the question. What about a later overdrive trans from behind a magnum engine? It shouldnt be hard to run one without a pcm.
The 46rh transmission can be used. The last model year was 96 on vans and 95 on trucks, latter models have the re transmission which has an electronic goverenor and needs the PCM.
Also on a van you need the cradle bracket as the transmission is larger and sor of the holes need to be welded and redrilled to match the crossmember. (At least they did on my 71.) Also the drive shaft must be shortered and the crossmember modified to clear the larger tailhousing on a van.
A change from 3.92 to 3.55 is only 10%. It can be done.
Changing the gears themselves require some special tools for bearing preload and backlash. Swapping the entire axle is easier for the laymen.
Either will require the speedo gear be changed to match if there is one, otherwise the speedo/odometer will be off.