Best bet is to buy a whole rusty 253 auto HZ.

Gearbox crossmember, tailshaft, speedo cable etc are the same. Hole in the floor the T-bar goes to is the same. Just needs the tranny and flexplate, T-bar, T-bar harness, console insert, vacuum line and brass T for the manifold for it, auto cooler or auto radiator and lines. Plus make a little mod to the dizzy vacuum advance hosing to get full vacuum advance at idle.
To the letter of rego laws you also need to put on an EGR valve too.

If it is a 3.55 diff which many HZ 253 manual were you'll probably want to look for a 3.08:1 diff as well.