Hi from the Dancing Wolf

Differences on the '72 and later 340's (nutshell version)
1) flat-top lower compression (8.5:1) as opposed to earlier 10.5:1
2) J-heads with 1.88 intakes as opposed to early J-heads and X-Heads utilizing 2.02 intakes
3) Cast crank, externally balanced damper and flywheel
4) 68-70 340's used Carter AVS carb, 71 and later used the Carter ThermoQuad
5) '68 340's had hotter factory cams than later years.
6) Water pump inlet and timing marks swapped sides in '72.
Basically these are the major changes...easily rectifiable with a mild engine build

Welcome aboard! Don't hesitate to ask any more questions, happy to help!