Sometime in the latter part of the '72 model run, they started using externally balanced cast crank 340s which required an external balance flywheel. If you have access to the 340, look at the harmonic balancer. If it's flat with no balance tab, you have a internal balance flywheel. If it has a raised balancing tab on it that says "for 340 external balance only", or something like that, it's an external balance 340 and you have a external balance 340 flywheel, only used for part of '72 and all of '73. As a last resort, see if you can google a pic of a external 340 flywheel to compare balance holes with.
