904/727 Flex Plate Question

Hello FABO!

I have a question about flex plate compatibility. I have a 318 and I am converting from a 904 to a 727, and I am noticing that the flex plate that is compatible with the 904 does not seem to fit the 727 torque converter. Is there a difference in the size and how can I tell what kind of torque converter I have so that I can determine which flex plate to buy? Thanks for your help in advance!