It's not the convertor, let me give you a method to try. First you are sure you have the "heavy" balancer, and I'm pretty sure you do, since your vibration is not real severe. I'm looking at an old flex plate, you have 2 of the convertor studs that are on both sides of the weight, run the engine and be sure you know how the shake feels. Now those two studs take the nuts off and put some 3/8 washers on, equal number on both studs, probably 3 will be the limit, you want to be sure the nut goes on all the way. We are adding weight in other words. After you do that run the motor, is it better or worse?? This should help to pin pont it.