Hi Fred, after watching your video I think that you should keep those parts and reattach them.
Fred, I am far from an expert, but I agree with Jimco. You need to keep them and replace any that are rotted.
Fred,I'm of the same opinion as Jimco and the Johnson's. It's not really a structural support but more a structure support. I'll explain; I believe they are there to add rigidity to the side walls by limiting the amount of flex in the frame when it bows front to back and also as a kick board for the interior walls. JMHO.
Well not meaning to rain on your parade but. My vote goes with the boy. I'd keep them there. My guess is they are there for support. While I hate to say it the factory put them there for a reason. I'd hate to see you put all that hard work into the project and then a month down the road when you tooling along the highway have a Awwwww S#*&t moment when you watch the side wall start flexing while at track speed on the interstate.