What Barry said for re-drilling and re-studding hubs. Wheel adapters are also available, but they add beam width, make the steering a little different and add stress to the bearings.
With 15 inchers, I would wonder about fitting a front disc brake caliper that severely limits the disc diameter size you could use. Bigger IS better! Same issue for any rear disc brake.
For clearance, suggest you take some plywood or heavy cardboard cutouts and bolt them them up flat to your existing hub. Then start taking measurements based on tire width and determine where you can put the CENTER of the tire-wheel combo. From there to your mounted cutout is your offset. Dont forget to look at full lock in both directions. Especially hardware clearance at front, inside of each wheel.
If your not endeared to your current fiberglass, you could always determine your new wheel center and simply scribe an arc on the existing fender (I did). Simply remove a bit of offending glass until you have no body rubs. Can't do that trick on the inside hardware when you're at full lock. That will probably be your limiting feature.
Finally you can be a bit consertive. When you're all done and you still want to move the tire/wheel combo outward - either for extra inside clearance or to move the rubber farther outboard, wheel spacers are readily available form several sources. You can buy them in l/4 inch increments. May be able to make your own.
Personally, I kinda like the look of a little rubber sticking out beyond the body line. Sorta like cleavage! Thans for the remind - I need to add a few spacers.