I think you are going to have a tough time finding these off the rack. Don't know if you could modify a set from lets say a Mustang with big block, or have to start from scratch.

You could probably start researching all the headers made to fit various Ford cars and pickups, and find something sort of close, then slice them up. Not really as horrible as it sounds, and sometimes the only way.

Also, on a convert you may be actually happier with standard manifolds and duals, and a whole lot easier.