Those D50 trucks have a really short nose on them. I put the sheet metal off of one on a Jeep J10 frame around 25 years ago. I needed to add 6" to the front fenders to fit the AMC 360 in it, and the radiator was still in front of the radiator support. Measure the distance between the radiator support and the firewall and see if your up to the amount of work its going to take to put that 5.9 under that sheet metal. Extending the length of the front fenders was the easiest for me back then. It was a ball of fun, but a lot more work then I thought it was going to be.

The trucks were originally Mitsubishi trucks, built in Japan, and used the same basic frame as all the Japanese trucks used at the time. If I remember correctly, the entire frame was a welded unit from front to back, nothing was riveted. It would be a pretty major undertaking to change a 4x4 into a rear drive. You would need a good front 2 wd drive frame to cut up, at which point splicing to the 4x4 frame wouldn't make much sense. I also don't believe the 2 wheel drive frame would accommodate the 5.9 magnum very well. I believe that was one of the big reasons I used the J10 frame instead of the 2 wheel drive D 50 I had. Gene