What 3spds manuals in 72 swinger?

No mounting pad for a floor shifter on a 903.

For that you need a 230 3-speed & then you will also have synchros.

Easy way to spot what you have, 903 is a top loader, bolt on top cover.

A body 230 has the bolt on side cover & a single mounting pad for a shifter.

The B & E body version has a longer tail shaft with dual shifter mounting pads.