rear axle

In the process of my 1985 90 rebuild I'm upgrading the brakes, intending to fit a discovery mk1 rear axle to get the discs.
I've stripped both axles and find that the 90 has fully floating half shafts whereas the discovery has fixed drive flanges, I've swopped the diffs and the 90 shafts now fit the axle but obviously I need drive flanges to fit. The 90 ones seem to have very slightly different hole centres to the discovery hubs, Discovery front flanges fit but are much shallower. Does anyone know what flanges I need ? Maybe defender 200 or 300 tdi ones ?

Is the Discovery axle 10 spline or 24?

If the axle you have fitted is 10 spline single piece shafts they can be either later flat type uncommon or earlier mushroom type shafts. You need to work out which type you have and get the correct shaft spline combo. It is also possible that an early RRC rear axle mis-sold as a discovery axle could be imperial.

Without a pic of the discovery axle we cannot really advise as there are so many options

If indeed you have an imperial case then Ashcroft sell the drive flanges to fit correctly. If you have fitted a 300tdi later axle flat/skinny drive flange then options are limited to rarer 10 spline 1 piece shaft. or refit the 24 spline diff and use original shafts for that axle. Your original shafts will be too long with later skinny hubs.