correct H pipe

As my brother and I have finnally reached the point of putting our car on the road for the rest of the season we have one final question to ask. We believe that our 68 GT500, Build date 12/6/67, was equiped with a 2" dual exhaust. To that end we installed the exhaust and H pipe that I believe was either purchased from NPD or Branda and found exhaust leaks at both exhaust manifolds. Needless to say we installed something wrong. What confuses me is that the H pipe came with what I can best discribe as a double flange as apposed to the standard single flange on the 2 1/2" exhaust the we removed from the car. This flange does not mate to the steel donut gaskets that the manual and the NPD catalog said we need. We therefore left them off thinking that this double flange some how replaced them. The question is, is the H pipe we purchased the wrong one.
As a point of clarification we changed the exhaust from 2 1/2" to 2" because we returned the engine to the 428 PI from the 428CJ that had been in the car when we purhased it.