Jeremy, Mopar as been offsetting the drive train in the frame for years, as have most manufacturers. Unless you have a really short driveshaft, the U joints can accommodate a 30 degree angle at each end, as long as the angles are not trying to bend the shaft into a U. That would amount to about a 3"-4" offset.
Usually, when you do a frame swap, you usually have to move either the cab mounts on the floor, or you have to move the cab mounts on the frame, and a lot of times, you end up moving both. The front cab mounts on a Dakota are way out wide, and most of the older stuff isn't wide enough to use the Dakota mounts in their original position. If, for example, your 57 cab is 60" wide, and the Dakota floor pan is 65" wide (not actual measurements) your going to have to trim the width of the Dakota floor to fit your cab, and what you trim off will probably be at the cab mounts on the Dakota floor. Very seldom is the rear cab mounts in the proper position.

Cab mounts can be pretty simple. You add a bracket to the outside of the frame, a 3" wide piece of angle iron will work, or a 3" channel iron will work, or even a 1/8" thick piece of formed metal with side walls and a top plate. The end of the bracket needs to stick out from the frame somewhere between 3" and 6" (though I've made them longer then that), and about an inch in from the outward edge of the top flat plate you need a 1/2" hole. You want that bracket the same on both sides of the truck, and at the same location on the frame. Then you need a rubber bushing, flat rubber, a block of wood, or an actual body mount rubber bushing (I prefer the actual cab mount bushings) that is at least 2" on diameter with a 1/2" hole through the center to act as a cushion. The top of that cushion needs to be above the frame, and it will sit on top of your bracket that is welded to the side of the frame. You need the place on the floor pan that lines up with the hole in your bracket to be reinforced, read that as at least double floor pan thickness, I use a piece of 1/8" flat that is larger then the cushion diameter. My preference is for that reinforcing plate to at least be tack welded (I fully weld mine) to the floor pan with a 1/2" hole that lines up with the hole in the frame bracket.I use a 1/2" grade 8 bolt, with a flat washer under the bolt head, push it through the hole in the floor pan, through the rubber bushing cushion, and through the bottom bracket. Then I add another flat washer, and a luck nut. Tighten the bolt until the rubber just starts to compress. You can get way more fancy if you wish. You can add a recess so the bolt head is below the floor pan surface, and you can add a rubber under the frame bracket. Actual cab mount bushings have a step bottom that would fit into a larger diameter hole cut in the frame bracket.They also have a steel sleeve the bolt passes through, and usually there is another piece of rubber that goes under the frame bracket with a very large formed steel washer. The actual body mount bushings cut down on the chance of the bolt rubbing on the sides of the hole and making noise. That also help to keep the rubber from being squashed too much and having it start to fail.

Frame shortening: All the area around the weld joint needs to be cleaned to shinny metal, inside, outside, for at least a 1/2" either side of the weld. You have to make sure both frame rails are the same length, and are square with each other. You also need to be sure the upper control arms on the Dakota front suspension maintains the same degrees of angle. They are not set flat to the earth, the front control arm bushing is higher then the rear bushing to get a proper wheel alignment and good driving traits when your driving. All frames have alignment holes formed in them. Their purpose to to get the vehicle pulled back to straight after a crash. Those alignment holes match the same location on both sides of the frame and there are 8 or 10 pairs of them on each frame. Find a pair on the front clip, well forward of where your splicing (or shortening) the frame from. There should be a pair just before the frame kicks up to accommodate the front suspension. Locate a pair well behind where your going to splice. Mark both pairs so you an find them easily. With the frame on stands (8 of them, 2 on each side of the frame cut lines, and two on the front section, and two on the rear section, once the frame is cut, the jack stands will be the only support for the frame sections) set it up so the rails are level. Check the frame for straightness, measure each side from the front alignment hole to the rear alignment on the same side, and write that number down. Now do a cross measurement from the front hole on one side to the rear hole on the other side. record those numbers. The measurement from the front to the rear hole on each side should be the same (+ or - 1/8"). Both cross measurements should also be the same (+ or - 1/8"). If they are not, you need to get your frame straightened before you cut it.

So the frame is straight. Lets say I'm going to shorten the frame 6". I will clean up the area that will be welded on both edges before I cut it. Then using a square, I will draw a vertical line at least 4" in front of where I'm going to cut and remove the chunk. Then, again using the square I will draw another line at least 4" behind where I'm cutting the chunk out. That is going to give me two square lines 8" apart, after the chunk is removed. I'll do this on both sides. Carefully measure out where the cut lines will be made. It may be best to leave yourself a little extra 1/*" off the cut line, then grind the extra off to be sure the cut is in the correct position. If the jack stands were properly place, once the first cut was made, nothing should move after the cut was done. Make all your cuts and clean up the edges. Slide the front frame and the rear frame section together. Make sure everything is still flat and aligned. At this point I will clamp a short piece of tubing or angle to the outside edges of the frame rail (on both sides), and I will also clamp a tube or an angle to the bottom edge. Then I will measure the distance of my drawn lines. If that is good, I will do the measurements of the alignment holes. If we are still good, tack weld all 4 corners of the splice on both sides, and check my lines, then check the alignment holes. Then I will make a series of short 1" long welds (the same area on each frame fail), checking my measurements after each pairs of welds. After a couple pairs of welds, I will remove my tubes angles and clamps. Are you getting the idea how badly I want to be sure is staying square? Repeat until its fully welded.
After the welding is done, and the frame is nice and square, I will probably also add a diamond shape piece of 1/8" plate (fish plate it) over top of the joint. The place I usually do my splice is at the channel section of the frame. Often times I will use the piece I cut out to box the channel section. Gene