I think the point of this exercise is to have duals....they were single with a crossover tube at the back,, originally. And using a pass side manifold on the driver side, the pipe would probably aim right at the steering gear, and you'd have two crossover holes to plug. https://www.jalopyjournal.com/forum/threads/cadillac-346-headers.306815/ ,
I suspect there would be little to gain...Except- If you did a little port work, bumped up the squeeze, ground a cam... What you building?