Stock headers are unequal "spaghetti" headers. Ford deformed the primaries quite a bit, so that they could run a large head bolt on the flange. Since you already have an intermediary, I would recommend something like a BBK unequal length shorties. I almost always recommend longtube headers, but that would mean replacing the intermediary in your case. BBK headers have a one piece flange that helps with gasket leaks. Don't bother with equal lengths. They are a huge PITA to install and deal with, and the horsepower gain is negligible.
Kurt