$15,000 is a lot of money but not unreasonable, if they are going to the job to perfection.
You could probably do the job yourself for half the price, including purchasing the TIG welder, but it probably wouldn't look or perform as good as one done professionally.
If you do decide to do the headers yourself, you might want to mock up the headers and piping in mild steel or cheap 304 stainless, before you commit to cutting any 321. Mandrel bent 321 stainless isn't exactly cheap...
...Mike
BTW: What are you going to do for a transmission?
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