They're saying "You're insane" because no 1974 340's were ever built. 1973 was the last year for the 340.
It was replaced in 1974 by the longer stroke (but smaller bore) 360, which had the same heads and intake system as the '73 340, but lots more torque due to the stroke increase (and 20 extra cubic inches.)
I think the cams were the same.
The (higher) early 340 compression ratio gave that engine more snap, but the 360 was probably a better street engine for the heavier 1974-'76 A bodies, delivering more torque for daily driving. The one-year-only '68 stick shift cam was also a plus, but was gone from the lineup all-too-soon.
I think that a '68 340 Dart (shipping wt. 3,038,) with a 4-speed manual transmission would be a really close race for a '71 Duster 340 4-speed (shipping wt. 3,140.) The better cam and lighter weight in the '68 might offset the bigger carb on the '71.