The early LA manifold on left side had about 90 degree turn at the bottom of it. One owner compared the restriction to pooping through a straw LOL. At the same time, a heat riser in the right manifold would close pushing more exhaust pressure to that left side, yet it seemed to work, good enough for a stock engine anyway. That 90 degree turn in the left casting disappeared for 66 or 7 but even the 67 commando exhaust was still a single pipe. From what I've read, only the Y pipe was a little different from standard 273 2 brl.
I think I would want a later left manifold without that kink even if not 340. The value of a good set of 340 manifolds is about half of that 800 dollar dual exhaust system. All just points to ponder. Good luck