The replacements from Mikes or the ones included in gasket sets are usually quite good. The collar at the head is quite good as well. We might be looking at "user error" here, lol. You did get all of the old seal rings out, right? Sometimes you can't even see them because they're so crusted over and partially burned.
Also, you didn't mention the type of exhaust you have. All stock? Stock headpipes with aftermarket mufflers? Complete aftermarket system? Problems can arise putting aftermarket mufflers on stock headpipes. The headpipes are double walled. There are vent holes down near the muffler and up near the head to let out condensation moisture from in between the pipe layers. If you just saw off the mufflers and clamp the new ones to the outer headpipe, exhaust gases can travel up between the pipe layers and sputter out the vent holes. It may appear that the headpipe to engine connection is leaking when it may in fact not be that at all.