I spent today installing a new catalytic converter I bought from FCP Groton for my '86 240 sedan and it was a good fit. I had had a crack in my header/down pipe so I pulled one from a low mileage donor car and installed new OEM Volvo gaskets for both. I started everything up and my car sounded like a rusty old red-neck truck without a muffler. I traced the exhaust leak to where the top of the header/down pipe bolted up to the exhaust manifold. I can feel the air where the two meet. It's like I don't even have a gasket in there at all. The connection between the front of the the cat where it meets the end of the down pipe has a nice quiet and air free seal. What did I do wrong? I removed the header/down pipe to check it for rough surfaces as with the exhaust manifold surface and both were smooth and fine. Any suggestions???
I made sure that my support brackets were not causing any flex or stress on the header/down pipe.

Thanks in advance for your thoughts on my problem.