if you could just stretch your stoker's inside leg measurement by about 1.5", I know where there is an impeccably restored Higgins Ultralite short wheelbase tandem looking for a new owner.
These tandems used a curved seat-tube in which the curve was fabricated from flat steel sheet strengthened with round steel bar welded along its edge. Yeah.. I know it sounds grotty, but Higgins were extremely accomplished bronze-welders., ensuring that the end product was always a thing of beauty.
I doubt whether there are many of these rare machines over in the States.
Norris Lockley