.. not sure if you were still looking for an answer to this question.
The metal dust covers on R160 axle output flanges protrude more than those designed to fit the R180 diff. The metal dust cover will bottom out on the side bearing holders (the alloy parts that hold the side bearings and bolt to the steel case of the diff) all before the long through bolt can correctly engage with the threaded retainer block that is located at the inboard end of the side gear (internally splined part).
The most common way around this problem when fitting R180 diff in place of a R160 is to carefully cut through the two spot welds that attach the metal dust cover to the axle output flange (couple of shallow hacksaw cuts will do the trick), and remove the dust cover.
On my rally cars, I always modified the dust covers and reattached them with a couple of spots of weld as they were originally, ensuring protection for the rubber side seal, which was Hitachi/Nissan's original idea. The down side of doing this is increased risk that a dust cover will be damaged, come loose and do even more damage ... your call!
Simplest fix is to carefully cut the metal dust covers off and throw them away, making sure you have not left any jagged bits to damage the side seals.
Everything works perfectly well without these metal dust covers, especially if a little excess grease is applied to the seal before reinstalling the axle output flange.
Note that if the diff (either R160 or R180) was sourced from a Subaru, there will be no threaded retainer block to screw into: Subaru diffs have internal "c" clips fitted to the side gears.
_________________ Things work best when it matters the least, and ...
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