There was a photo of a dual Weber downdraft setup on the old Yahoo club. He had to fabricate it himself if I remember correctly. I always thought it would be too much carburetion for the little A-series, but we know what happens when I start thinking. If the owner of that setup is here, he could give you some tips on that. Sorry, I can't remember who he was but I believe he was in southern California.
As for the rear bar, mount the thing to the leaf springs. Clamp it as close to the axle as pooible where it bends forward at both ends. Then clamp the ends themselves up near the front eyes of the leaf springs. You shouldn't need to weld anything.