His best guess was a 7/16 nut. Couldn't get my stock nut to thread on a 12mm bolt.

Gets better though...find out my driver side manifold has some heat riser emissions deal in it. This makes it so a later model h-pipe won't fit. Looks like I'll be upgrading to later model manifolds/headers. Anyone have any laying around that are in decent shape?

Feel like I've opened up a HUGE can of worms with something as simple as exhaust. I don't think I can use my old bolts as they are all rounded off. I'm crossing my fingers that I won't need new studs. If I need new studs, I'll have to remove the manifolds Taking off the manifolds means I run a very high risk of snapping off one of those old, rusty header bolts in the head. This car will now affectionately be called "Pandoras Box".