Head replacement, hit a snag

[Mr. BILL]

Everyting has gone well, except with one of the exhaust maifolds. The three nuts on the underside of the manifold won't come loose. I took it off with the head. However, the size of the nut seems to either have corroded to the point that it's no longer a 12mm like the others, or it's a bastard size. 11mm, is too small, 12mm is too big and 7/16 is too small. I tried PB Blaster and heat with a pair of vise grips. No go.

Since I'm not worried about the studs, should I grind them off? Sugestions would be greatly appreciated.

Also, is there something I can use on the aluminum patrs, like the timing chain cover, t-stat ect. to make them look newer? I tried a brass wheel, but that's too coarse.

Thanks