[Steve Dickey]

Jason,
For what it is worth I'm holding an exhaust head stud for a 427mr head. The half that threads into the head is a 3/8"-24 , the other half is a 3/8"-16. I suspect it is the same as yours. If not, a good starting point. You may want to try heating up a wrench and bending it to fit around your foot box if you don't want to cut a hole in your box. Anyway you slice it, it is a bear to get to.
Good luck, Steve