Here's what my book shows:
340-ci V-8 (available 1968-73)
Bore......................................................4.04 in.
Stroke...................................................3.31 in.
Compression ratio........................10.5:1(1968-71)
.................................................8.5:1(1972-73)
Carburetion.....................single CarterAVS 4-barrel
Horsepower...................275 @ 5000 rpm(1968-71)
...........................240 @ 4800 rpm (net, 1972-73)
Torque..................340 lb-ft @ 3200 rpm (1968-71)
......................290 lb-ft @3600 rpm (net, 1972-73)
So it looks like the factory 340 went from 68 models to 73 models so likely they could have had a casting date as early as 67 for the 68 model cars. The 360's became available in 74 and ran to 76. Now this is A body info so other body styles may be different.
While we on this here is some other interesting data.
1969 Dart Swinger 340...base price....$2836
1971 Demon 340..base price.............$2721
1970 Duster 340.......base price........$2547
Production #'s
1967 GTS............457
68 Hurst/Super stock Hemi......80
69 dart swinger 340......16,637
71 demon 340.......10,098
74 dart sport 360....3951
70 duster 340.........24,817
71 duster 340.......12,886
75 duster 360......1421
76 duster 360........N/A
Thought this would be of interest to some of you. Too bad we didn't buy a barn full of these things back when we were kids. When i was a kid i could have afforded these to a tune of one a month but women were sure occupying my mind at that time. Now when i'm older i only have about 40 of em so i guess i'll just trudge on through my boring old life. I guess life is tough everywhere though. Yall have a good day and vote (#,#,,,/#,,,,) tuesday.