If you don't have a thin enough wrench to fit the flats of the brake bolt, try what I do when I initially install calipers....loosen the mounting nut until the caliper rotates freely, I usually don't have a brake cable installed yet, but it shouldn't matter here...grab the brake arms and squeeze them tightly against the rim as if the brake is applied. The return spring is now loaded equally and the arms can't rotate. Start tightening the mounting nut as you hold the arms against the rim. Once tight, release the arms and the caliper is pretty centered against the rim.

BTW, those black plastic things are bushings to reduce friction where the spring touches the caliper. See pic above, just slip them back on there. You don't want to toss these!!