You will spend more time making all the little brackets and spacers than you will putting the main frame together. I've been debating whether to make an entirely new frame, which would fully take advantage of my front end, or modify the hell out of an existing one, which might be considerably easier. I hear of people welding stuff onto stock frames. All my frames have been made of tubular steel, and I don't see a big need to do any aluminum spar concoctions. Wholeheartedly agree with you here.
Tube frames are the way to go, and steel is the material for tube frames.
Michael, I am planning to build a single in a TZ250 frame. But I don't yet have the frame, and welcome alternatives. I've just finished the Husaberg as you probalbly figured out when I went bonkers over the proposed Supermono weight limit. This would make it weigh in at less than 220lbs - wet. I think you're right about steel.