You are going to have to go to a wrecking yard, used car lot, stop a stranger etc. and take some measurements of the frame, or simply commit to spending the money and picking up a Dodge frame and Toyota X-cab(if you don't already have one) so you can have them side by side to to see what it is going to take to get it to work.

I don't think either is going to work easily, and will likely take a lot of fabrication to make it work. You would save a lot of money by either installing a toyota 4cy diesel into the X-cab, extending the frame and using a 7' 2wd bed, or picking up a T100 x-cab w/bad motor and installing the diesel engine you have into that. The T100 frames were built the same way as the other trucks, i.e. fully boxed and ladder frames. The only difference between the Toyota 1/2 tons and the1 tons was the springs. The Toyota frames are likely to be just as strong or stronger than the C channel frame that came on the Dodge.