A '72 Challenger 3-speed would be an A-230 & that is not one of them.
You have a A-903.
The 230 has a removable side cover, the 903 has the removable top cover seen in your photo.
The 903 was used in column shift '3-on-the-tree' cars so it has no factory mounting points for a floor shifter like the 230.
3 on the tree was never available in any E-body, stick shift on the floor only.