I am a bit late in joining this conversation and I assume you have made your decision, but for what its worth here are my thoughts.
It sounds like with that many hours the wet kit must have been used alot in order to have only 370000 some miles on it, assuming that the hour meter and odometer are both correct.
I am with the majority here and say overhaul the C15. I wasn't sure you could still get a cat in a Western Star in 05, but like Mike said, a truck that is speced that way with double frame, 46000 rears, wet kit, 14000 steer, etc. is not easy to come by and Western Star has always made a solid cab.