I had that setup on my old '78. It mated no problem to the stock manifold. There was an issue reusing the stock gasket. The original piping was not flush and thus the gasket could "rest" on the lower pipes. On the twice pipes, or at least mine, there was no extra piping and the gasket is literally held there by the pressure of the bolts when you torque it down. Might want to use some sealer to insure a good, snug fit.
Other than that, I don't know where you would put a cat. The system goes from stock manifold to a 2.5 collector/merge pipe and then out to the dual pipes. Unless you hack into that collector there really is no where to put a cat. To my knowledge, no one has twice pipes AND a cat.
I'd run it straight and if its too loud for you, talk to an exhaust shop. Maybe you could fit some other glasspacks along the duals...hmm....
If I were you though I'd save for some headers....it is MUCH deeper with the 3-2-1 which I'm running now. Not sure how the 6-1 and twice pipes sound though.