1999 Infinity QX4 Catalytic Converter/Exhaust Manifold Join

Yup. Flange with a lip that interlocks with the manifold. There is also a "wire washer", and a cap, which is a hollow ring that covers the wire washer, that act similar to giant "o" rings within the cats. Leaking like heck!

The theory was that because the manifolds are new, and the cats are old, they may not ever be able to seal because the did not warp together. But that doesn't seem right to me, because that would mean that every time a manifold got changed, the cat would have to change, too.

That's probably not the case, since BOTH manifold/cat connections are leaking. In any case, the mechanic is claiming that the cats (not replaced) and the manifolds (new) would never give a perfect seal simply because the old cats would be slightly warped just due to regular use. That doesn't seem to me to make sense, since that would mean that one would ALWAYS have to replace cats along with manifolds to get a perfect seal. Besides, shouldn't the wire washer/caps take care of this by acting as a gasket?
The short question is: should I be able to get a tight seal using the old cats along with new manifolds? Is this a common problem? Thnx.