I'm going to back up a bit now. As the body and the chassis merged, one area of concern was the very rusty cowl and the cowl vent that always leaks. Also, all of the coupes windshield wiper system was gone.

The Dakota has a pretty neat setup. The top of the firewall has what I'm calling a vent box. Inside the vent box, the heater fresh air and the cab vents come off the back of this box. The complete wiper system is housed in the vent box, and there are water drains on both sides of the cowl. The vent box is completely closed off from the cab except for the fixed mentioned duct work, and the vent box is above the heat/ac unit. If I can transfer this vent box concept to the coupe, I might actually be able to have dry feet while driving the coupe in the rain, not something very common with this era Plymouth.
Since I'm using the Dakota firewall, I know the width is close. Since the cowl already has rust holes in it and the vent area is really in sad shape, I figured I didn't have much to loose. So, I started off cutting the cowl off the coupe. I cut just above the firewall seam at the front, along the rusted out sides, and a couple inches under where the coupe's wipers would have been. Sorry, I thought I had pictures of the process, but nope. Sorry.

Anyway, after I cut the cowl off, I made a template from cardboard. There would be a one piece back and bottom, but since it needed a crown so water could drain out both sides, the back would have to be split in the center. The bottom front would weld to the firewall. The top back would weld to the cowl, near the wiper location. I could build both ends and close in the center as required. I wanted about an inch and a half drop on each side from center. The back panel would need to be as vertical and flat as possible other then the edges and center.

I discovered that with the cowl removed, it was a lot easier to fit the Dakota heater in the coupe. With the exception of the fresh air inlet, the heater/ac under dash unit fit snugly in the coupe. I had to "clearance" the area around the fan housing and trimmed some of the firewall upper structure from the passenger side that used to support the hood hinge. While I had such good access, I made the heater/ac unit fit. I would still have to make a fresh air to recirculated air door area to completer the heat/ac unit at a later date.

Once the heat/ac unit was bolted to the firewall (in the original mounting holes) I returned to the vent box fabrication. Once I was confident with the cardboard template, I transferred it to a new piece of 18 gauge steel.
The vent box floor/back wall was bent at about 80 degrees, before the back wall was cut at the mid point. The piece was bent in the center at about 20 degrees to form the water shed. Once satisfied with the fit, the front and top was welded to the body. The center was enclosed, and the ends were enclosed. The passenger side was enclosed with a couple pieces so the heater fan housing would not be in contact with the firewall/ vent box. Care was also taken to assure water would be able to drain away from the box without sitting anywhere. i have pictures of the finished box.