... Last few ones I have done have been converting good disco boxes to defender spec using worn out / water damaged defender boxes as spares donors.

To do this swap everyone will tell you the box needs to be stripped down totally to swap FTC4588 involving hunreds of pounds of labour. It can however be done in about 30 mins from start to finish. Drain the box, remove the bellhousing (six bolts), and top shifter mechanism. Remove the bolts holding the alloy rear housing and cast iron front housing to the mid plate and remove both housings. You are now left with all the gears and shifter forks held in one large cluster around the centre plate.

To remove the shifter rail locate the shift pins and measure the depth of protrusion through the shift rail using a vernier. Invert the gear clusters, position the shiftrail so that the shifter pins can be drifted out individually - they are an interference fit and the shift rail will need to be supported, easier with a helper to position the cluster while you drift the pins out; I support the rail around the pin using an old socket. Once the pins have been remove the old shift rail can be pushed through the forks and the new defender one fitted, making sure the detent doesnt fall out. Refit the shifter pins to the correct protrusion measured earlier. Reassemble box fitting defender shift quadrant instead of disco one and defender shifter mechanism.

I have seen disco boxes with the end of an LT77 shift rod with quadrant welded to the disco shift in situ and using the lt77 shift mech.