By the look of the picture, those nuts haven't moved in a long time. Expect to break afew. I was lucky enough to find mine had previously been replaced with stainless studs & nuts. While that welshy looks to be the culprit, pressure test the cooling system anyway.
A tip for anyone loosening exhaust studs/nuts/bolts, use a decent amount of penetrene, leave for a few hours, lube again & then nip the nuts/bolts up before attempting to loosen. If they move, then you have apretty good chance of loosening without breaking. If it breaks, well it would have broken if you tried to loosen first.