I'm pretty sure when I did mine it gave 85mm I'll have to recheck it.
When you put the axle under the springs the easiest way is jack up the rig, remove the wheels, disconnect hand brake cable and drop the springs, well one end anyway, axle just rolls out , you just have to measure the position of the pack plates on axle, remove them and fit on opposite side of axle, reposition spring and bolt axle on under spring. I actually fitted another plate in case I want to return it back as an over spring. Ideally use new U bolts and fish plates but at least use HT washers not the mild steel jobs that seem to be used, they distort. Readjust hand brake cable.
You cannot just rotate the axle or rotate the brake unit as the backing plate only goes one way and it's a lot easier to relocate the small plate.
Also if your axle has been aligned using the plates or axle bending then you should keep the same aspect if your measurements were good enough.