The stiffener plates are after factory parts added by the outfit that did convert conversions for Ford. To add subs I think it is best to:
- Put the sub in place and mark the car very carefully
- Drop the sub down and pop (drill the heads) on the rivets along the uni-frame
- Use and highspeed cutter to cutback the stiffener plate so it butts to the edge of the sub per your marks
- Put the sub up and weld...to frame...to stiffener and tack stiffeners along the body as well to get a reasonable cross brace for free
Just my two cents on how mine went. Whatever you do weld them and enjoy