I stripped down and repaired an early box,they can't be swapped with each other.My '70 bay now has a late chassis front rail so I had to get a late steering box as the early one would no longer physically fit.
I found though, that if you have too much play in the worm drive bearing in the early box(it wobbles from side to side) then you can adjust this out(splitting the whole thing apart and removing the peg arm) then remove the bearing end plate held with 4 bolts and remove 1 shim,then refit and test again.It should rotate freely without play.Then refit the peg arm and rebuild the casing and adjust the locknut for play in the central position,rechecking once fitted to the van.
I think the earlier ones are more easy to maintain but you can't get the parts.The late ones weren't made to be stripped or maintained so easily apart from the locknut adjuster.You just replace the whole thing when it wears out.
I now have a spare late and a spare early steering box which I can't sell incase I need them one day.