Thanks for the details and photos.
After looking at some tire / wheel calculators, and for what it's worth ...
IMHO you've done a good job of specifying the tire size / profile that satisfies your goals
Personally, I'm still concerned about the tire / fender (and flare) clearance.
You want to fill the fender with a tire that will definitely extend outboard beyond the fender / flare line. The obvious potential problem is rubbing. You're wanting to cut it close (with respect to fender / flare clearance or gap), and I'm not sure how close is too close for comfort.
I estimate the 255/60/15's will reduce the gap between tire and fender / flare by 7/16" - 0.5". Do some measurements / eyeballing to see what losing 0.5" of clearance will look like and if you think it's no problem.
One thing to bear in mind is that if you damage or break one of those SR flares (e.g., by bottoming out in a pothole) you'll have a helluva time replacing it.
I checked for a slightly lower profile 255 / 15, but as far as I can tell nobody seems to offer one.
One other thing ... Shifting the wheel parameters and tire height as you specify is going to affect your handling and ease of steering, because it will increase your scrub radius in the positive direction. I don't know of a good way to predict what will change or how much it will change. All I can tell you is that it *will* change.
As long as you're intending to use the SR for basic street / highway cruising (with no hard cornering, carving up twisty roads at high speed, or off-pavement use) I suspect your planned setup will be OK.
Good luck with it, and please report back to us here on CT how it turns out.
(No warranties, express or implied ...