a518 Vibration

Hey guys, I recently got my car back on the road after doing a bunch of stuff to it. It now has a rebuilt 400, A rebuilt A518 with a JW Ultrabell, Brand new driveline and a rebuilt 8.75 amongst other smaller stuff.

I've noticed that the car has a vibration in 1st-3rd (before going into OD and LU). the best way to describe it is as a rotational vibration. It mostly goes away when the car goes into overdrive and the TC locks up while cruising. At risk of poisoning the well of answers (so to speak), I'm wondering if I have the wrong flex plate? Could this be causing my vibration?

Thanks in advance.

Grant