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Head to your nearest truck stop or Peterbilt/Mack/International/Kenworth dealer/parts store. They should be able to lead you in the right direction for parts and install. Good luck.
Make sure you put a weatherstack on the top to prevent rain from pooling in your exhaust. If you do not, it is possible to get so much water in there that you will blow apart your exhaust and grenade your muffler trying to start the truck. At least on the big rigs this happens. A flexible elbow seems the best because it ends up being a week point, plus, you can remove it easily to get something out of the exhaust too, like a small animal.
I would make sure I put the heat shields on the mufflers, though they tend to be expensive. They will help keep heat from radiating from the mufflers to the truck and prevent serious burns or fires.
You can get some light gauge steel 90s at a plumbing store, hardware that kind of thing, you'd probobly want to take it out to what, maybe 4" stainless for the part you'd actually see? You could put a fastback on your truck and have the stacks come up through there, then top them off with some popcorn flappers. That would be killer. I'll bet you could do the whole thing for under 100 bucks if you could find somebody to weld it for you.