Not trying to be a male member either, but if you are that well versed in auto body and repair, you know that the ONLY frame that fits under a late 30's mopar is a late 30's mopar. The guys that got a clean sheet of paper in the early 80's to design a new mid-size truck did not try to make a frame that would fit under a car built 50 years prior.
That said, the Dakota is fairly close, and someone with your ability should be able to do what's needed to make it work. There really isn't any other Mopar frame that has the track width, engine/trans placement and flat midsection it does.

If you look at the frame under the 38, you will see that it doesn't really need boxing like the same era Fords, unless you are going to install a 500+ HP motor and drag race it. Take a look at the other threads here and you will see some others asking the same questions, and some answers. Unfortunately, no one has documented the whole process yet, which would be great for everyone. I hope that you can get your shop built and get started on this project. It would be really great if you could take pictures, write up the process and document how to do this; it comes up a lot!