To look original or factory you need a late ute cross member to swap motor.

With gearbox if it only has the one gearbox mount then in a 1200 Auto tunnel you will need to make/extend the existing gearbox crossmember about 15-20 cm some boxes have 2 mounts one near tailshaft input then you only need to extend crossmember about 20mm .

As said diff is a straight bolt in get it especially if it has a removable inspection plate on rear and borg warner written on the bottom side near where the tail shaft connects.

Get all the front end from 120y especially the castor/radius rods and sway bar.

You will need to swap the springs and strut tops off the 1200 struts on to the 120y ones as the 120y ones will make the car sit higher and throw things out a bit you will see the difference when you have both out.

Recently with other members advice i removed the second shortest leaf spring from the 120y pack and added it to the top of the pack in 1200 flipped over.
Did it while i was swapping to the above mentioned diff. Makes it nice and stiff and also will sit lower cause it flattens your springs.