Ballancing a 1973 340

If the inside of the engine is all original than you should be OK. I assume that is what you mean by 'stock'?

The damper you show from Summit has 2 external weights that can be bolted on with the supplied bolts; you would use the smaller of the 2 weight for the later 340. There should be other dampers that are good for this.

For the flywheel. there is a weight kit for the McLeod flywheel that you show, PN 564111. But if you are using a different and neutrally balanced flywheel, then the procedure should be to find what weight can be added to any neutral balance flywheel or flexplate for the late 340, and place it on your flywheel at the same distance from the crank centerline, and at the same angle relative to the crank bolt pattern. The info to add such a weight to a neutral balance flexplate is out there, but I just don't have it handy. Maybe someone will chime in here with that info.

If your present flywheel is already imbalanced then the sure way to redo it is to first neutrally rebalance it, and then unbalance it with the right weight for the late 340.

And note that adding a weight at a certain distance and angle on a flywheel or flexplate can be achieved just as well by REMOVING the same weight at the same distance from centerline but at the OPPOSITE side of the unit. That is the principle at work with items, like the B&M 10235 flexplate as one example.