Depends on your budget??. The 340 driver and passenger manifolds of the 60s were both 2 1/4" outlet, they were used on the 340 6 pack. The 70s 340 manifolds were a mix of the 340 driver (2 1/4" outlet) beefed up to help avoid cracks and allow for spark plug shields. While 70s 340 passenger manifolds were only 1 7/8 outlet, which made no sense because the driver's manifold was 2 1/4" outlet.
In my own opinion, the 70's 340 driver paired with the "machined" 360 magnum is the best of both worlds. You get the flow of 2 1/4" outlet for a better price. The 360 magnum also exits in a much better place than the 340 passenger. The 360 magnum doesn't crack nearly as easy as the 340 passenger (much thicker ears) and cost less to replace (more readily available) if it does crack. That's the only reason I started selling these manifolds. I prefer the 70s 340 driver because it more beefy and because the 60s driver is harder to find anyway. The only advantage to using the 60s 340 driver is that it fits the EARLY A bodies where the 70s 340 doesn't fit without major mods.
