904 transmission rear seal

Ok I'm new to this so bear with me I have a 1972 duster with a 340. I bought the car which was restored six years ago but now have a rear transmission seal leak. So I went to change it and the seal listed is too big compared to the one I pulled out. How can I find out what this transmission came out of this car is a clone and originally a slat six car. It seems the tail shaft is much smaller than what's listed for the duster