Yes, this is correct. One side is a 2 1/4 flat flange and the other is smaller with a ball socket type flange. Im pretty sure this was the case on the 71's as well. If you want the big 2 1/4" opening on both sides you need to shell out ridiculous money and get some 68-70 340 manifolds.
For a mild street driver I would send the 72-3 manifolds. They flow decent