I have the 80" WM Zero which is a great flyer. I assume these are simular however yours seems to have better outline and detailing. I used the standard aluminum tube "as is" and have had no problem. A solid dowel at the tail would give additional tap depth for screw which holds stabs in place. Personaly I believe the wing tube as is will work just fine but heck the I beam is a great idea and the weight won't hurt this planes flying one bit.
I did epoxy my tube into wing for a perm. one piece wing, same at tail and I would not do it again - I think I would prefer the seperate halves for easier transport.