Just so we are on the same page, your 1973 340 should have came from the factory with this HB (which is for the cast crank).
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Next, your torque converter should have came from the factory with weights attached as the cast crank requires this. You can use , or so I have read, a neutral balance converter (no weight) with a cast crank motor if you get the correct flex plate from (say) B&M racing. A lot of converters now have no weight, Chrysler sells the weight and a template so you can weld it on yourself.
I guess I am not understanding what the shop did. If they balanced everything, as written, there should not be an issue.
Since there is an issue, something is not compatible. Not quite sure but I will keep thinking about it.
As for your questions, I have never tried that, and would be hesitant to do that; but that being said, I would defer to those with more knowledge on this.