HELP!! Torque convertor and flexplate issue

SGBarracuda is absolutely right. Those weights on each side of the drain plug are balance weights to balance the externally balanced engine and the flexplate with the cutout does the same job so that throws it out of balance. It don't matter if it doesn't shake at only 1500 rpm, it's freakin out of balance, that's why it shakes at other rpm's

JFYI: you are correct that they do have to balance the converter after welding it together but the weights to neutral balance the converter are around the perimeter of the converter and their long thin weights, not on the front side by the drain plug. Also the bolt circle is not symmetrical co it'll only bolt up one way. I've noticed the B&M plate covers the drain plug. No idea why they designed it that way.