I don't know where your getting those part numbers from but you want:
Part #3DFS727A

Automatic 3-Speed 727 18" Tailhousing.
Replaces 18 1/2" tailhousing.
60's and 70's muscle cars and street rods.

http://www.gearvendors.com/aghr3s.html

Are you sure you even have a 727 in your car?

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