I also had issues with an exhaust kit I ordered for my 59 from one of the major suppliers. My kit included the type of flange with the donut gasket, and moveable flange where it mounts to the exhaust manifold, instead of the fixed flat flange. It appeared the kit had a 64-66 part. I called and two days later, I had the correct part, but when I turned sharply to the left, I could feel binding in the steering, as it was also hitting the steering arm. My mechanic eventually took that off and altered the bend to clear.
The 2nd issue was that the exhaust tips were for a 61-63 Tbird, which would have sent the exhaust discharge into the bottom of the rear bumper, instead of the 90 degree turn downs it should have had. They eventually sent a replacement for those as well. The major suppliers get these drop shipped from an outsource, so when I called, they had never seen the parts anyway....at least they were willing to make it right, but it was frustrating to get it all done correctly.