See number 20. Does it have a head for a socket? I just see a round button.

My #4 nuts have welded itself to the flange, should I shear it off and punch out the bolt or drill through the bolt?

 

Those flange bolts are studs. Soak the nuts with PBBlaster and heat them red hot with a torch. They'll most likely come off. There is no way they got hot enough to actually get welded to the flange. They are just rusted and can be broken. And if the stud breaks you are no worse off than before. Again, red hot and Blaster should do it.

Rich

 

I didn't have a torch or other fancy stuff, so I cut mine off (sawed them off) partly by hand, partly with a saber saw (hey, you use what you got After a bit of sawing I would put a wrench on them and I was finally able to break them off that way without having to saw all the way through.
Forgot to mention, saw them off from the nut side.