Considering magnum swap

There's two ODs with two types and I think 4 strength varieties out there. (3 500s, 1 518- like with the 3 904s and 1 727) There are non-computer controlled ODs. Some of the 518s are non-lockup. The 42RE/RH is often what you see with V6 OD setups. Pretty sure the 42RH is the A-500 in many of the '89+ 318 trucks/vans. Same bell pattern, neutral balance like a 318- lighter duty, 904 based. The 44RE is commonly seen in 318 OBD-2 apps. The 46RE/RH is the 727 based trans, the heavy duty variant for V8s. Vans are weird since they had 3spds optional for years after trucks- there are OBD-II 3spd vans of about any variety from V6 to 360.

There's so much cross-compatability with 5.2s and 5.9s that if you later decided you needed a 5.9 PCM to go with the 5.9 there's very little you'd need to change besides the balance parts. With 3.91s, I think you'd be pretty happy with the 318, especially if you put in an OD to lower the revs on the highway.