Are there any signs of balancing on the crank journals where weight is removed?

It is possible to internally balance a 340 with much lighter rods & pistons than OEM.
Are the rods 340/360 sized or the earlier, less beefy, 273/early 318 rods. Any grinding/polishing?
What brand pistons? Forged? Part #? Weight?
Any machining in the undersides of the pistons?

If you can't come up with examples of significant reciprocating weight reduction over OEM, taking the parts to a machine shop to check the bob weights & overall balance may be the only way to be sure if it's done correctly or if you will have a vibration.
Was the engine run at all or was a fresh rebuild/overhaul?
It's possible that the previous owner had a vibration after the rebuild and sold the engine because of it.