I haven't tried a "dry" exhaust (small boats have a water jacketed "wet" exhaust) but am a retired boat mechanic... most everything could be counted on to be quite rusty. By all means, use plenty of penitrant, a little at a time. Six point sockets is also a good tip... If the American size is loose, go to an undersized metric and hammer it on until you get one to bite. Put a breaker bar on it, not a rachet, until you get it started. Try loosening but also tightening. Give the breaker bar a wack with a hammer in each direction to jar it... then oil it again. Repeat both until you get it. Don't get anxious... give the oil plenty of time to work.
PS Many times, after a job, I'd have to put the socket in the vise to drive the bolt back out of them with a punch... Buy Craftsman sockets, when ya break 'em they're guarenteed.
2001.5 Dodge Quadcab, 2500, 5.9, AD150, big line, 47RE auto, 4X4, 4.10, 265/75R16, 8800 GVW, TCW3 4"TBE, 36" FTE swirl resonator, straight through "muffler", IAT (mpg) fooler, XZT+. NRA LIFE MEMBER