You might just insert the new half shaft, install one of the clips, then use a feeler gauge to measure the gap and buy a spacer just a bit smaller. You could either measure between the clip and the edge inside the diff or maybe even the gap on the outside of diff between seal surface and half shaft edge.
Below is the gap that seems to vary from one shaft to another.

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Jimmy L.
'05 Acura TL 6MT
2001 ML430 My Spare

Gone:
'95 E300 188K "Batmobile" Texas Unfriendly Black
'85 300TD 235K "The Wagon" Texas Friendly White
'80 240D 154K "China" Scar engine installed
'81 300TD 240K "Smash"
'80 240D 230K "The Squash"
'81 240D 293K"Scar" Rear ended harder than Elton John