Your mock up looks like you’re going lowboy. I’d have made the rails if I wasn’t doing a highboy. I’d probably just do it with rectangular tube rails and maybe some “dress up” sheet metal skins from the fire wall forward to give the rails some contour for a lowboy build.
That body looks solid.
I have a 30-31 five window body but I need to pretty much re-make the lower six inches. It probably sat on dirt for a long time before I got it. Originally I was going to make a chassis similar to my roadster’s but a lowboy would save enough time to push me in that direction. They look good either way. Keep the photos coming.