This is going to be a major undertaking. If that 36 frame is still good and not rusted away, updating the brakes on it would be much easier then what you are considering. Once you get newer then about the 41 model year, that Dakota swap is much easier. The front frame on a 36 is pretty narrow, and the widest part is at the center, but the rear is not much more narrow then the center. A Dakota frame is wide at the front bumper, narrows at the front suspension, then gets gets slightly wider just behind the front suspension, and remains that width to the back bumper.
The 36 has a very narrow grille, radiator & radiator support, and the hood at the grille. The fenders are huge, a lot of frame can hide under the fenders, but the narrow grille, radiator and front of the hood are the biggest issue.. If you get much wider then the 36 was originally in that grille/radiator area, it flat kills the appearance of the car.
the oldest Dakota swap I've completed was a 39 Dodge pickup, and a 48 Plymouth business coupe, but I've done around a dozen Dakota swaps. The wheel track (the width from the center of the wheels as mounted on the rotors) will put the tires right at the edge of the fender edges on that 36. That would be with using the Dakota front wheels, the Dakota wheels set in at least an inch more then a standard wheel. That 36 body is wider at the rear then it is at the front, using the Dakota rear wheels will put the wheel almost against the inner wheel well on the 36. It would not be a surprise if one side of the 36 body was closer then the other side. When you set the body on the frame to build the body mounts, measure very carefully, be sure the body is centered over the wheels.
The Dakota used the 5 bolt wheels only up through the 1990 model year, in 91 the Dakota used 6 bolt wheels. I would suspect that if it really was a 318 truck, it is a 91 or newer Dakota. Swapping to a 5 bolt wheel on the 91-96 Dakotas on the front is a simple rotor swap. The rear is more of a challenge and involves axle and drum bolt pattern drilling. Past 97, its all a major challenge. The 5 bolt wheels were the standard Mopar bolt pattern of 5 lugs on a 4 1/2" bolt circle, so most of the older Mopar/ford mag wheels will bolt on. That is helpful on the rear, deeper dish wheels on the rear improve the inner wheel well clearance.
I did a build thread here titled "a 49 Dodge on a Dakota frame". I went into detail on that project, much of it is going to apply to this project.
The wheel base on an extended cab Dakota may be too long for that 36 coupe, but the Dakota chassis are very easy to shorten the wheel base on. Expect to cut off the front of the Dakota frame just in front of the coil spring pockets, anything forward of that will have to be built from scratch. Also expect to cut the rear frame off someplace behind the rear leaf spring mounts.
I would suggest you park the complete Dakota side by side with the 36 coupe and do some measuring before you cut anything off anything. Line up the front wheels of both vehicles with the tires pointed straight ahead.
The 39 Dodge pickup has the same narrow grille/radiator issue your 36 is going to have. The 39 was fender less, so its easy to see what you are going to have to deal with.
Pic 1, The 39 Dodge on a Dakota frame.
Pic 2, The grille and radiator support are probably pretty close to the same width as your 36. Notice everything in front of the spring pocket is outside of the grille/radiator. Also note that everything in front of the spring pocket had be fabricated from scratch. With fenders most of this can be covered.
Pic 3, This is a 5.9 magnum (same outside dimensions as a 318). The real purpose of this pic is the very bottom is the clearest pic I have of the spring pocket, fabricated forward frame. and the grille mounting. Gene
