Seems to me I'd want a new case, not a welded or junk one- the junk yard one has a real risk of the same thing happening if there has been any excessive pressure inside it prior to it being junked for other reasons. Not to say it has had a presence of overpressure, BUT do you really want to put a used case and new PRV in your tractor only to have it fail (possibly) not too far down the road? It seems like a $ and frustration nightmare waiting to happen. Then you have to do the same thing all over again?!
I'd want to go with new and peace of mind for the extra expense. JMHO
BTW, that's why they are called junk yards. Your decision- obviously, good luck with whatever you conclude- just playing devil's advocate here to level the choices out.