If as you say above the flexplate is from an AU its going to be a 50 ounce one. 289's used a 28 ounce one and there is therefore a 28 ounce balancer on the other end of your engine.
I'll be corrected if I'm wrong but I'm not sure you can just change the balancer to a 50 ounce one as the whole rotating assembly has been set up to have 28 ounces swinging off both ends, not 50. I think you could get your flexplate rebalanced to 28 ounces.
Why does it need an AU flex plate? why would not use a 28 oz Windsor plate and if you need 164 tooth you just use a Cleveland plate. Thats what I did on an AOD. You can re balance a 50oz plate to 28 however if needs be. like you say you cant fit a 50oz plate to 28oz rotating Assembly its a different crank. Flex plates are cheap though like $100
