There was a degree of commonality between the late RRC and Discovery I that may lead you to some parts that will join things up for you. Some late RRC did use 24 spline shafts.
You'll need to look at some parts books to see where you stand in relation to these. There are a selection of drive flanges/hubs which might fit but I suspect that you may be on your own in sorting this out. You'll certainly have to do some measuring to work out spline engagements. I don't believe that there is anything in hand to have less than full engagement into the diff side gears.
Be a little careful with spacers. This bolted joint transmits all the wheel torque and every extra interface that you introduce reduces the 'quality' of the joint. If you can get spacers to work dimensionally, bolt it all up and weld the spacer to the drive flange which will effectively eliminate the extra interface.
It would be helpful if you could identify (s/n) the recipient RRC and the donor Discovery - assuming that the parts are original. Someone may have more joined up knowledge than I.
Of course, the diff core and bevel gear/pinion are fundamentally the same regardless of the halfshafts so you may be thinking that you're getting something better when in fact .....