and then the holes welded or patched up in some way? Has anyone tried this? I couldn't even find heat exchangers in Pelican's catalog.

it's just metal...

I believe they are NLA from any source except used. Mine were bead blasted and welded up, so the answer is yes.

Cool. I thought I might have to look for a more expensive solution.

Eastwood sells a stainless steel high heat paint that works great on clean rust free exhaust. I've had it on three vehicles for over 5 years and it looks at least as good as some ceramic coated headers on my k5 blazer that are about 5 years old.
It's supposed to be baked on at 400 but I used a heat gun and a infrared thermometer gun and it seemed to work ok. On a short system you may get by just running the motor to bake it. Thats what I did on some vw headers. It needs to air dry about two days depending on how thick you put it on. It is really hard to clean the spray gun up afterward though. Don't plan on spraying any clear after using the ss paint or you will get little sparkles in the clear. Don't ask why I know. But my hd tank has sparkles in it.

I've used the stuff I've mentioned in this thread and am quite happy with it...... Ceramic Paint Certainly much cheaper than Eastwood and possible the same stuff.