The fundamental imbalance of the rods and pistons is 100% the issue. I just ran the numbers: By going from stock 360 LA rods and pistons to KB107's and SCAT rod, the bobweight has dropped by approximately 300 grams versus stock. That is massively off. So unfortunately, this is going to have to come out.
You will indeed have to take the crank, rods, locks, bearings, rings and pistons to a shop to have the assembly balanced, along with the flextplate and present damper as a check. I think this can be balanced as an internal balance system; if so, you can remove the external weights (remove the weight on the damper and change to a balanced flexplate and TC).
- I would personally prefer to do this as an internal balance system; the extra external weights just add some extra stresses on the crank that can be eliminated. If this is a cruiser car, then that is not so important.
- If the external weights are maintained, then it will require extra metal to be removed from the counterweights on the crank. This might become a practical problem, by making the end coounterweights into swiss cheese!.
- FYI, the bite out of your 360 flexplate is equivalent to a weight on the other side of the flexplate.
The crank balance to your measured piston/rod assembly bobweight can be achievable by taking off weight from the present crank, regardless of whether you end up internal or external balanced. So that is good.
I cannot comment much on the shop situation. I am actually quite amazed at the reaction, but do not know the whole situation.