So when i bought my car it came with the header in the pics (which bolts to the main cat), no idea what it is , pacesseter??? custom build?? im not really corcern about that other than i have a leak and is from the union the header and the cat. as you can see in the pics, the end of the header is not smoth at all and i want to know if this is what is causing the leak, and how can i fix it.

 

I would say that the flange's uneven surface is causing your leak for sure

get it machined down - or even grind it and check the flatness with a straight edge - get a new gasket and re-install.....