Once the box is off of the frame, the frame is easily welded by a qualified welder and will be completely serviceable. The plate was probably done because you can't really weld a trailer frame right with the box on it without burning it down...
The OP didn't add any color about how he got here or why he's considering restoring this trailer, but with 75% of the floor gone, at least one wall sagging, and I can almost guarantee roof damage as well, it's almost a total rebuild from the frame up.
jayfraze wrote:
Probably 75% of the floor is rotted or soft which is something I've dealt with before
This one might be better off as a part-out, post some pics when you have time
