1974 - 904 Transmission Reverse/Low Band - ???

So I'm getting my 904 Transmission rebuilt that came from a 1974 Plymouth Roadrunner 318. The problem is I can't seem to find the correct size of reverse/low band, the used one is currently bigger than what I'm used to seeing in other 904's, can someone pinpoint me in the right direction of where I can get the correct band? This transmission is going behind a 318 Magnum in my 1965 Plymouth Barracuda.

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