The 340`s made from 68-71 had the 10.5-1 compression as well as larger valves, in `72 the compression dropped and the valves got smaller with the advent of more strict emission controls. You can do a compression check and (granted the valves are in decent shape) get an idea of what compression the engine has. My stock `68- 340 with original pistons runs in the upper 170`s.