71-73 left side manifold had a 2.25 inch outlet, right side 1 7/8 outlet. But the pipes on both sides were only 1 7/8 (or 2 inch?) for the first couple of feet, then fed into 2.25 inch pipes. Not only that, but the driver's side pipe was indented for torsion bar clearance.
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As for performance, I'm surprised someone hasn't posted the Mopar Muscle (I think) dyno testing of a 300 hp crate Magnum engine with different exhaust. According to them, the best 340 exhaust manifolds were only marginally better than LA 318 manifolds, and headers were only maybe ten hp better.
To that I would just cite my own experience: Back in the late seventies, I put a set of cheapie headers on my 73 Duster 340 that was stock except for a 71 intake and a 650 spreadbore Holley plus the factory heat riser was removed from the stock passenger side manifold. I didn't replace the rest of the exhaust system, which was completely factory original down to the original mufflers. The headers were worth seven tenths of a second in the quarter. Actually they were worth more, but now it had so much more low end torque that now I had a traction problem that I had not had before.