340 vs 360 confirmation

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

I picked them up yesterday, along with a set of '75 high back bucket seats with tracks in very good condition , the stainless drip rails and wheel moldings (all in excellent condition), hood hinges, AC compressor brackets, hood bracing, an emblem or two, and a few other detail bits I was missing, all for almost nothing. Just a local Mopar guy, "friend of a friend" - now a friend after this past week - who was looking to help out instead of get rich. I'm amazed how supportive Mopar folks are of each other, in person and here.