Nigel,
Glad to learn you have stopped smoking.
Original driveshafts can be replaced with “modern” production items with CV joints as a straight swap without having to make any mods to other components.
You will not have to remove the gearbox but you will need to split the lower swivel joint to get clearance to extract/refit the shaft from/to the hub. With the shaft out you can then access the joint at the gearbox to remove the inner yoke.
B…..
Bernie