i have swapped an early Rover (1950s) 4.3 pinion and crownwheel onto a late 4.7 metric carrier when the need arose, the problem with early parts is mostly to do with bearings being imperial and the case being machined to suit the smaller bearings
so i'd still check out Crown Differentials website as the chap that owns it has set up a nice explanatory page on parts/building and swapping/adapting the Rover internals to fit the different cases
Rover diff pinions are shimmed to set the height so possibly be prepared to at least buy shims if you build your own Rover diff rather than buy one complete ready to just plug in
a little extra that you might want to know is the 4x4 ability can still be used even with odd ratio diffs as long as you are on a loose or slippery surface (not hard packed or metalled) as the loose slippery surfaces allow the wheels to spin up freely as needed to keep away the transmission wind-up, i still have a 4.7 diff in the front of my 109 and have used 4x4 without the world exploding a couple of times, i've even used low ratio on the road a couple of times but with my FWHs set to freewheel to let the front halfshafts spin free of the wheels (DO ENSURE they're set to freewheel on metalled/hardpack as i won't be held responsible if you break something by following my experiences)
you can also run with the hubs engaged to lube the swivel pins regardless of the surface as long as you DO NOT engage low box or 4x4
i hope i've made the distinctions clear enough