Update: I was able to use the brakes - didn't notice these were a later model with integrated bushings. The bushings are floating on the arms and fixed to the bosses, so there's no problem if the bosses are shorter. However, the bosses' "shoulders" were 1.5mm too long and were filed down for proper spring insertion.
Now I consulted a Shimano manual and found this:


My rim is 24mm and boss spacing is 78mm, so it's just at the X/Y area border. The required L1 distance can be achieved with the thick washers outside (L1=41mm) or inside (L1=60mm). What would be better?