[350TSS]

Exhaust studs - wire brush the exposed thread to clear any rust on the threads (small wire brush on a Dremel can be useful here) Then daily treatment with a 50/50 mix of acetone and diesel for a week then heat the stud as close as you can to red heat ( a creme brulee butane kitchen torch is great here about £6 on ebay) in the hope that the stud comes cleanly out of the head. With a very good fitting socket try tightening a fraction first to see if you can break the rust bond between the nut and the stud or stud and head or even between the nut and the flange then reheat and reapply the magic solution then try loosening. If it moves re-tighten then re-loosen - repeat, repeat, repeat in small increments. Bloody difficult to fit a socket to the rear head exhaust studs, maybe need to sacrifice the ring spanner end of a combination spanner and grind the outside until it fits between the exhaust pipe and the nut.
If it does not move, try carefully splitting the nut with a small slitting disc on a Dremel, the stud will need replacing anyway but you want as much stud showing as possible to get the remains out of the head.
If the stud still does not want to move you can (after disconnecting battery and earth leads to the frame) weld another nut onto the recalcitrant stud. This often works as the weld heat transfers down the stud to the thread in the head and breaks the corrosion bond.
Good luck