Dan, basically the issue with the flexplate or torque converter is that one should be neutrally balanced and one should be balanced to counter-act the dampner.

The B&M flexplate is said to be for a 727. Should work on a 904 as well, but I haven't seen it even attempted. The easiest way to take care of the balancing problem is to balance the balancer to the flexplate and then get a neutrally-balanced converter, since they are more plentiful. You COULD get a torque converter balanced to the dampner and use a 360 LA flexplate, but that's usually more time and money spent.

Let me know if you need anymore specific information. I am almost through my research for the article (serious research going into all of the options) and would like to have it done by June. I will let you know so you can archive it.