Your very lucky. Thge sytem on my 2001 dodge is shot already. the tail pipe after the muffler has already rotted off and required welding to re attch. The rest of the sytem isnt any better and willl need replacing before next winter.My 1988 dodge p/u has a stainless steel exhaust. It is magnetic so I assumed it is a 409 type.
Also keep in mind that increasing your wall thickness decrease your heat transfer rate. From what I have seen and people I have talked to stainless wins hands down. off course stainless is no replacement for a good design. The round designs tend to have te pooling problems that Crofter mentioned and typicaly rot out from the botom from ash, and cresote holding in moisture. The heatmore I have is designed in a way that the bottom of the fire box doesnt hold any water. Basiclly the botom is a fire brick plkatform with a cast iron grate in the bottom for ash removal. The wallsand the roof of the box are the only parts that hold water. Ill see if I can post some pics to clarify tommorow.