Like Robert says, they're essentially the same size, both will fit. Things to consider with a V8 swap into a 4WD:
- To keep the mechanical fan, you need to replace the hood, front clip (what the radiator mounts to), grill, and bumper from a 91 -96 Dakota.
- If you go with an electric fan, no changes needed to the body, but you'll need a "pusher" style fan that fits in front of the radiator. That's what the Shelby Dakotas had.
- If you use a stock LA motor you might be able to get away with using your V6's transmission. Magnums being more powerful are too much for the transmissions that were used with the V6's.
- Your V6 transfer case will work with most Dakota V8 transmissions from the 90's, including the 5 speed NV3500, which I'm using.
- If you use an LA motor from the same year as your Dak, you can run the TBI fuel system with minimal changes. If you choose a Magnum, you'll need to decide if you want to run it with a carburetor, the Magnum Multi Port Injection, or something else. MPI will require changes to your engine bay wiring harness (pull one from a later Dakota), and a different fuel pump and tank.
-stock Dakota V8 oil pans from the 90's work in the 4WD, no problem.

 

What Ragged said, however, I will disagree a bit on the transmission. The V6 transmission is an A500 which is an 904 with an overdrive. Many strip cars pushing a 1000HP run the 904's. I'm not suggesting slap it on with no modifications, but rebuilt properly, the A500 is capable of supporting a Magnum. It's what I'm doing in my build.

 

Yeah, I may have over-stated the case a bit with respect to 904's. I broke one in a '72 Duster I owned many years ago and so there's a little bias. I always stuck w/727's after that. Actually the 904's (A500's) built since the late 80's may be a different animal than what I had.