New update, for the one person who cares...which is me...
I got the spacers and I tried them out last night. The wheels fit. Yay! There's plenty of thread for the lug nuts to grab.
I ordered 185/60-13 Sumitomo HTR-200 tires this morning and they should get here by Monday. I think in a straight line they'll fit just fine, because the overall diameter isn't that big a difference. Turning is another thing.
Really, I think the only time the tires will rub is at full lock. And full lock on a Justy means it can pivot around a rear wheel. That's not often a useful ability, so I'm not worried about losing it.