Catalytic converter question, 83 body, 78 frame drivetrain

[teamssc]

I've just have had my truck a few months and I'm trying to budget for an exhaust. The one on it is really rusty and loud. It might be just the mufflers and some gaskets but I've got to get under there and look closer. I started calling shops today to get a phone estimates and they are from $350 to $500 or more if it needs cats. There are no cats now. I've read a lot of different threads in this forum and others and I can't nail down if they are required. The body and the vin is an 83 C10 diesel but the frame, engine and everything lower is a 1978 K10 with a 350. The shops are telling me they have to follow the EPA laws and put in what was original or get fined. Over the phone they won't take the time to 83 diesel vin is required to have a cat. I've read different things. If the 83 diesel vin warrants a cat but the 78 engine etc doesn't I still don't think I have a leg to stand on to avoid the cost of the cats but I might find a sympathetic ear at a smaller shop.

So the questions are
1. Does the 83 diesel C10 require cats?
2. If it does to I have a prayer in hell demonstrating to a shop it's a 78 frame/engine?
3. Someone told me the frame has it's original 78 vin stamped on it, if so where? Would that be enough to convince a shop it's a 78 or all they going to go by the titled vin? (I think the answer is the body/title vin dictates)
4. How reasonable, with limited welding skills, would it be for me to put in my own dual exhaust, or do I just suck it up and do whatever the shop tells me? I might also have a friend who is good at welding.

Thanks for any help