Plumber, If you have 4-1/2" grinder get a couple thin (0.045" thick) cutting wheels, cut the studs off at the nut, heat the nut red hot, and apply candle wax (paraffin) to the nuts after they cool a little, re heat the nuts to melt the wax. Do this several times than heat them up and try to remove them with a box end wrench.
I have had a few nuts that would not break loose and did this trick to break them loose.
Watch out as the wax creates a lot of smoke when applied to the hot nuts.
Another trick is to just heat the nuts red hot and try to remove them.
When you assemble the new fasteners use copper anti-seize on bolt threads.