You will more than likely want to connect that downbrace....there's a reason you don't see them floating like that...they are a major structurally important piece of the frame and unless braced otherwise, I cannot see how it's going to be ridable. The hard tail is typically joined there, the seat's hinge often welded on (in your case a bit lower which would apply lateral force onto it,) and it supports the engine. Looks cool in the drawing, would look cool in real life, but would likely either crack your frame of ride like a fish out of water.
Not trying to be a buzzkill, but just thinking realistically here. Besides that I like it. Look up Chapelle Customs "stout 33." Your drawing reminds me of a stretched version of that...also you may enjoy Jdizon's boardchop thread....very similar idea.