Kroil, made by Kaino Products I think, is liquid wrench X 10. In my 35 years of wrenching nothing compares for quick removal . . . except the flame wrench.
In an extreem case, drill the top of the bolt off, remove the plate and weld something across the top of the bolt. Use the kroil along with modest wapping with a striking tool, give the penetrating
oil a chance to work. Several mins. Wap a few more times while gently working the bolt both directions, while applying more penetrating
oil.
Good luck
Chuck