Converter hub and splines changed in 1968. Slant six and small block. Need to make sure converter is neutral balanced. Don't know if all 340's are neutral.
Greg, if the engine/trans is 1968 or newer, just go ahead and use one of your race converters.
Late style front, and early splines is easy to get done. But it is not necessary to change converter. Just put an adapter bushing in the end of the crank. Going the other way (early front, late splines) is more difficult. Not many converter places will do that.
