Say you had to drill out a exhaust manifold bolt completely out. Could someone use a bolt nut and lock washer to replace the one bolt?

 

I sort of did that with one of my cars when I replaced the stock manifolds with aftermarket headers. I'm referring to the flange studs that the catalytic H-pipe would attach to. The stock manifolds had the pressed-in studs, while the headers had holes that required using bolts. That should work fine, as long as you have a way to hold the bolts steady on one side while you try to tighten the nut on the other. If you're drilling out the studs from the factory manifold, you could drill the holes slightly smaller so you have to press the new bolts in.

 

if you have a flange off the head the manifold bolts to so you can get a nut and washer behind it, yes it will work fine.