I am also a Caddy fan. I'm stuffing mine in a 49 Ford truck. There is nothing better thatn big cubes and tons of torque.
Yes, the Eldo has the same transaxle as the rivvy and the toro. Its a TH425 I think. As far as I know the flexplates for all of the Caddilac trannys were all the same for those years, frt wheel drive or rear wheel drive. I think the Olds flexplates carry a different part # but I don't know how much different they are. A local parts store might be able to help you out with it.

Good luck on the T bucket, it sounds like a fun ride