You need to go back to basics here. Check that the battery is connected firmly and that all the connections and low-voltage leads that power and trigger the starter motor are in their correct places and receiving power. Don’t forget to check for earth straps and cables that might have been left off in the reassembly process, too.

If the car is getting power (and the dashboard lights suggest it is) then there’s a problem between the ignition key and the starter motor. Tackle it methodically, checking for power (a test lead or multimeter will be invaluable for this) as you work your way towards the starter motor and you’ll probably find it’s something really simple and basic. Make sure the fuse or relay for the starter motor hasn’t been fried during the gearbox swap, too.

The standard differential on this vehicle was a non-LSD (non-limited-slip) unit with a final-drive ratio of 3.08:1. Some owners changed the gears in the diff for more acceleration, although this was more commonly done in V8 models where owners were chasing performance at the expense of a little fuel economy.

If you’re in doubt, you can put the car on a hoist, mark a spot on the driveshaft and then turn the rear wheel a full revolution. By counting the number of times the driveshaft rotates during that process, you’ll know the ratio. In this case, one full rotation of the rear wheel should equal 3.08 rotations of the driveshaft.

Yes, it can be rebuilt or replaced and an automatic transmission specialist would be able to do that for you. Cost for the job would be around $2000 to rebuild or replace the transmission on a changeover basis.