When it came to performance parts I have always had the best luck by sending back the part to the guy who built it. Then asking for an estimate before making repairs. You will probably get it fixed the least expensively that way.
If you have lost confidence in the person that built it I would explain that to them before shipping it back.
I have had converters break and sometimes they were fixed for free and sometimes I was charged. In some cases the explanation of the failure was great. I felt good about the repair and why the part broke, and my confidence in the part was restored. In other cases I just got the part back, sometimes with a bill sometimes no charge. No explanation given or an explanation of we didn't find anything wrong with it. That did not restore any confidence for me.
This is just my .02 cents. I hope you get the part fixed and are happy with the final outcome.