Honestly, I would try to make up the 20mm with wheel fitment. The whole thing we're trying to avoid here is having to shorten the housing and axles. If you're shortening anyway, you may as well start with a better diff (meaning something with the correct stud pattern, disc brakes, and LSD options).

If it helps though, having just done a 'guesstimate' measurement the splined section is approx 32mm long & of that about 80% looks to be the engaged area.