1970 340 Balance Problem...Bottom Line

I will throw in my 2 cents,factory a 70 340 is internally balanced as it should have a forged crank.The harmonic balancer would not have any counter weights and would be similar in design to a 318.If it is a internal balanced engine then you should be able to run a off the shelf convertor but if its balanced externally the best bet is a aftermarket flexplate that will eliminate the need for a external balanced style convertor.What makes this difficult is all the possible scenarios,I mentioned before that Ive had many aftermarket convertors off on the balance and vibrate,Ive also seen engines improperly balanced and cause vibration so its hard to pin down.One test is to lightly powerbrake the car and if the steering wheel shakes it usually indicates a convertor problem-usually.