66 Hipo Manifold mismatch/serial number?!

[thecerealcommie]

So I was under my car, poking around and I discovered a rust hole in the bottem of my Hipo manifolds. This would explain why I have such an exhaust odor most of the time.... Anyway, I don't really know how fixable something like this is, so I order a new reproduction set from NPD. When they arrive, I notice quite a difference - First off, the new ones are almost 3 times as heavy, definately made of cast iron, whereas the old ones appear to be just thinner. They are less "elegantly" made, have beefer material around the head mounting holes, and are about 3/4" short of being able to mount up with the existing exhaust flange. Furthermore they boast being cast with the correct number:
C3OE-9431-B (driver side) wheras the one I took off had a C3OZ-9431-B number.

So my question: Were the ones I took off cheaper repros from a few years ago? How do I make them mount up to the existing pipe? I'm considering cutting the pipes further back (in a straight area and making a cheap patch get me to an exhaust shop.

Suggestions or insight?

Thanks,

Aaron