The RFE doesn't need an adapter. I have one that I swapped into my Dakota (along with the 5.7 Hemi) and it uses the same BH pattern as all of the other common Dodge transmissions. The output is also the same- 23 spline, same as many other Dodge transmissions/t-cases. Its also exactly the same length as most other autos, including the 42RH that was in my Dakota.
With that said, I have already almost drowned in wiring while building the harness for this engine/trans. I'm only able to do it because I'm keeping the package. I would never, ever try to use the RFE alone! Although some early versions used a separate controller, its VERY intertwined with the engine controller. It needs to read such items as RPM, ignition timing, and many other things to work. Its not as simple as adapting a throttle position sensor to the diesel.
The easiest thing to do would be to use one of the Dodge diesel transmissions, such as the 47RH used in the 90's (such as my '96). It is controlled by a computer for overdrive and TC lockup, but you could use the diesel computer and adapt the throttle position sensor to the 4bt. The 47RH is as much junk as the rest of the Chrysler autos, but it should hold up much better behind a 4bt than they did in my Ram (lost count of the number of transmissions, but this one was slipping with under 20k on it, installed by a dealer)
Jim