Serro Scotty: convert to overlap joint and J Rail?

jbae, I also have a Scotty, have nearly completed a frame up rebuild, and in the process of installing new front, back and roof skins, have also contemplated doing what you suggest. I have the original old side skins back on and will be doing a 6 piece replacement of new skin for the rest. Because of some structural improvements over the original build I am also hesitant to do the edges the way Scotty did them originally but... our walls are only 1/2" thick, unlike Shasta and others, and nailing or screwing on Jrail offers a great risk of penetrating the walls on the inside, and the thought a million protruding little sharp points is off-putting. Short enough screws or nails that won't poke through would not, in my opinion, give a satisfactory hold to the rail to the side and that is why the original build used cap rail, where the fasteners go through 3/4" material.
My plan is to install the new aluminum with an overhang on the top and upper front and back after fastening it with staples into the thicker 1X material and then trimming it about 3/8" wider than the sides, and bending that down. I have done a couple of test pieces and that works OK on straight runs but leaves something to be desired on the curves, where it seems like it will keep the cap rail from getting a good seal because of the folds you have to make.
Whatever I decide to do I will also use "seal tite" tape:
catalog.sealdesignllc.com/product/1-25-seal-tite-corner-seal-tape/
on the edges where the walls meet the front and back so that in the rare chance that my Scotty gets finished and towed out of our yard, rain or any water being flung at it at speed might be slowed down from getting in.

That's my short answer to your idea.

Nothing to add with the exception of Welcome to the site. Danny