Just to wrap up things here I ended up using the following parts to install the catalytic converter.

Cat to passenger manifold -> No seal.
Cat to crossover pipe -> Seal ring (part # 202-492-02-81)
Cat to mid muffler (driver) -> Seal ring (part # 202-492-02-81)
Cat to mid muffler (passenger) -> Seal ring (part #126-997-00-41)
Cat to mid muffler (passenger) -> Flange

I also ended up needing some bolts as they were really rusted and had to be cut out.

Thanks for all the help.