1972 GM 350 throbbing engine vibration

[Franks 72]

I have a 72 Suburban with a stock GM 350 (1st oversize) with a strange vibration. Vibration is not really severe but is annoying and varies with engine RPM. I have disconnected accessory belts (alternator/water pump belt , PS belt with no change. I am wondering about the harmonic balancer. Engine was completely rebuilt with the only change being 1st overbore ,slight cam and new aftermarket harmonic balancer ,aftermarket polyurathane engine mounts. When replacing the harmonic balancer I was told by the parts guys that the original 8" balancer was not available anymore and would need a smaller version which is aprox 6 1/2 ". In retrospect I wonder if this is correct and is this the source of my troubles or will harder rubber in the engine mounts translate into more vibration being passed to the frame.
Any advice would be appreciated.