Mine were tight, and because of where they are, it made it hard to actually reach up to them and still get much force behind the ratchet, but I did as Makeitfit suggested, and did them up 1/4 of a turn, then wire brushed the thread and put molislip on the thread plus a squirt of the Lidl's rust eater spray.
I recently had to change the rear section of my exhaust, and the nuts on there were seized solid, even the air impact driver could not undo them, and ended up completely rounding them off. I broke my nut splitter (it was an old cheap one) and the nut was in a recess, so I could not get on it properly. So I used my Dremil with the large cutting disk to slice the sides of the nut on both sides. I was not worried about the bolt, so could go deep, but if you went just clear of the thread, you could then give it a quick knock with a small cold chisel and it should open up enough to get it off.
