904 output seal

After far too long, I'm diving into fixing my puking transmission. Output seal boot sliced open, pretty easy diagnosis even for me. Problem arises as I get the new seal to press into the case, and won't even rest in the lip. A904 transmission I believe to be original to the 72 swinger original slant six car. The new seal is bigger, and the boot is shaped slightly different. tried all the parts shops, rockauto, napa, AZ, OReilly. Everything listed for the 904 is that slightly bigger seal. Am I missing something? Or off chance the trans isn't true to the car, would another generation be the key?

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