Charlie - Every set of Fentons I have ever seen are as I described in my earlier post. I have about seven sets in my possession and they all are flat on the front header and a beveled gasket surface on the rear, but that is really not the point here. The point is that to use Fentons on a six, you have to use a donut gasket to allow the exhaust pipe (the pipe from the header to the muffler) the ability to move a little. The donut gasket allows that to take place. As I said, if for some reason the headers you have are flat surfaced on BOTH of the headers, there is a flat surface doughnut that is made and can be used to allow the head pipe to be attached by being flared and ride on the doughnut gasket with the pipe being held by a pull up flange, not a bolt up flange. That arrangement allows some movement for the head pipe and allows everything to be slightly "loose" instead of rigidly tight.
Again, I will say, experienced exhaust installers know what I'm telling you to be the best way to install these type of dual exhaust systems. From the comments here regarding "other" ways to make the system function correctly prove that you can't just "bolt them up" and expect no problems. Mine have been on my truck as I describe here for 30 years with no problem. Right now if I had to move one of my head pipes over 1/4" because it was hitting the side of the oil pan rail, I could loosen the flange, move it over and retighten the flange and the head pipe would be over and stay over. The don't bolt up "tight". They need to run a little "loose".
