Prior to your post, the last post on this thread was 5 years ago. So yes, it was dead. And your post isn't even about a Rhodes Car; you're just thinking of making your own multi-passenger velomobile.
One of the problems with the Rhodes Car was that a steel frame that big but built to any reasonable weight is going to be too flexible. If money was no object, carbon might offer hope. Triangulation and depth-of-structure will be your friends. Use fat tires for shock absorption.
Keep the weight down further by using mesh seats like Andrew uses at RecycledRecumbents.com. And keep the seats down low for aerodynamics, not like the Rhodes Car which put the riders so high they could see over small buildings. And far from finally, do some research into building a freewheel-based differential so you can drive both rear wheels without scrub destroying your maneuverability.