57Vintage, thanks for sharing your plans! So from the outside you will see a new piece of trim running all the way around the trailer between the shell skins and the belly pan?
Not necessarily. The red line represents altering the bottom edge of the shell if it extends below the C Channel. If the edge of the shell is even with the C Channel I'll add a piece shaped like the red line, but slide it behind the shell piece. It would be like a Z channel you place between two pieces of stacked paneling on the side of your house. It's placed behind the upper shell piece and covers the top edge of the lower belly pan piece. I have both situations on my 57.
In either situation there would be two horizontal rows of rivets. One on the bottom of the shell pieces into the C Channel and one below that to hold the top of the belly pan pieces to the added square tubing welded on top of the frame outrigger. That would be the most visible change.
One additional item I came across this morning that I plan on adding; a drain from the C Channel through the square tubing and outrigger into the belly pan. See post 21
I plan on using aluminum tubing instead of the stainless that perryg114 used. I'm hoping this will minimize damage from any leakage in the ceiling or walls. I don't plan on insulating the belly. I haven't decided on the type of weep holes to put in the belly pan pieces so water doesn't collect against the steel frame.