340 engine vibration

512Stroker said
If the timing mark shows up on the passenger side that is a balancer for a pre 1970 engine most likely a forged crank internally balanced.
Your 1971 engine is a cast crank and most likely external balanced?
This being the case you have the wrong balancer?
Time to do a bunch of homework.
2 major configuration of the SBM 340
Someone will chime in to give you better guidance

Good point about the TDC mark on earlier balancers. However OEM cranks were forged in 1971 and I believe started to go cast late in '72. Cast will have a thin parting line on the casting and sharp corners on the counterweights. Forged has about a 5/16"+ wide machined parting line with rounded counterweights.