Your car is about change point between A type and J type.
find whatever is a good price.
you need a relay and loom for an A type
you need spacers for the cross member and longer studs for a J type
get a saloon box, TR ones are more expensive anyway.
different back plate I think.
Saloon flywheels are great big heavy things. Perhaps get a few kg shaved off.
OD gear sticks are not cheap usually, so perhaps find another solution.
Very early (63/64) gearboxes may be different, but don’t think so.
Long back and Short back refers to the crank.
Mk1 2000 and MK1 PI are long back, rest are short back. Get the right flywheel. You need a short back one.
make sure you fit a Mk2 type release bearing carrier and slippers, what ever gearbox you get.
fit a new cross shaft pin, and cross drill it as well if you can. Also drill hole in bottom of cross shaft pin hole in fork, so you can get snapped pin out when it breaks again.
finally, Lockheed clutch masters are expensive, and use a series 3 Land Rover slave. Much cheaper.