Balance Problems...Vibration......

If it's a '70, it should have a forged crank. If they used an aftermarket piston andd did not re-balance, it will vibrate just like you say. If it's a cast crank, without the right matching dampner and convertor, it will vibrate like you say. A cylinder misfire will not get better or worse...just speed up with rpm. In any case. Do Not run the engine any more. If you feel it, guess what it's doing to the engine's internals and block. I would start with pulling the oil pan and taking a good look-see at what you really have. Also, the balancers BJR speaks of are what I use too...And I just got one for an LA engine shipped from CA to CT for $70 new in the box.