Congrats on finding a Dakota donor.

My best advice is to pull the front sheet metal off the Dakota and park it side by side to the C, line up the front wheels. That will enable you to determine where everything will have to be moved to. When you start moving stuff, its best if you can make the firewalls line up as well. You will probably have to cut a notch into the "C" firewall for engine clearance. It makes doing anything to the motor easier if the notch in the firewall clears the entire motor. The firewall and floor pan are easier to adapt then relocating things on the motor. Any mid 50s Dodge cab I've worked with I had to notch the firewall. Depending on how deep your notch will be, you may be able to use the Dakota floor pan and firewall, but it may not be worth the effort.

The 39 Dodge pickup I'm currently building with a Dakota chassis has the motor & trans shifted back about 6". I just added a 1/4" thick plate under the motor mounts and moved them back. I added extensions (angle iron) to where the trans crossmember bolts to the frame to support the crossmember.

What I have found is that when you move a motor & trans back on the frame, the effect of weight on the front suspension has little effect. The V6 front springs can support a lot of motor when the motor is moved back.

The right end of the Dakota HVAC box is the recirculation part of the box. It can be cut off, and moved out towards the seat, (cut off with a cut off wheel, add a little flat plastic riveted in place, and a bit of sealer and its good to go) that will shorten the box about 6" in width. When you get to that point, I believe I have pictures of how I did my coupe, that should apply to your truck.

The Dakota steel box floor is wider then the step side box, except where the wheel wells are located. The Dakota wheel wells are less then 48" apart, so a filler panel (about 2" wide and the length of the wheel well) would need to be added to both sides at the wheel well. The Dakota rear frame kicks up over the gas tank, so the box floor is high. Your step side box may have pretty short sides, if you add wood on top of the steel, the floor will be even higher.

The last words of wisdom (for now): If you have the space, don't scrap anything until you are driving the new creation. Otherwise, you will scrap something that was just what you needed. The remains of a Dakota carcass can be an infinite supply of odds and ends that can far exceed the scrap value. Gene