Balance Issue / Vibration

Is it possible for a forged crank 340 to get rebuilt with the wrong crank/damper combination? I know stupid stuff happens all the time, but I am afraid some sort of FUBAR like this may now be what I am faced with. The engine in this car was rebuilt before I bought it, at least that is what I was told, and there is a re-paint showing. Runs great. But (big but) It seems to have developed a vibration issue now with the new automatic.

Block casting is a 68-73 340, by the driver side number. I understand the later blocks were cast crank/external balance. I can see from the paint it was painted orange over the original blue. Blue would be a later cast crank, right? It has no weights on the vibration damper, just some dimples I assume relate to the stock internal balancing process for a forged crank.

This comes up because I was doing some function/leak testing after getting the new 727 installed and it develops a very noticeable vibration starting around 1900rpm. I'll know for sure next week once I get the driveshaft in and get it on the road. But it should not do that, even just running free, should it? It has a neutral flexplate and 2400 stall 11" converter with no weights.

TIA for any thoughts or advice.