Mike,
If you're not ready to drop the motor into a completed Cobra, I would suggest waiting to see if the supply chain issues improve. If the motor is holding up the completion of the project, consider trying to locate cheap low performance original cast iron heads to get rolling and swap them out down the road for more desirable units. If you haven't tried already, look on the Ford FE Forum,
332-428 Ford FE Engine Forum you may find something there.
John