The wheel itself at 15" diameter is not where you will encounter problems. It is the offset (in/out) that can cause the wheel/tire to hit the suspension or fender. While it is true that with the 15" wheel you will likely have wider/taller tires the Pinto wheel wells are rather generous with space.
I've included a picture of my wagon that for a short time used the '88 Turbo Coupe 16", 225-60-16" wheel/tires. There was very minor rubbing on the front fender near full lock and I had to space the rears about 1/4" outward to clear the leaf springs. Also included is a shot of 225-60-15" on a set of 90's era wheels. I called them the "Carl's Jr. Star" wheels.

Both were temporary as the tall tire really bogged the car and did not look good anyway. The ride suffered too as it seemed the tire was just too stiff. I am now running 175-70-13" on Mustang II wheels - again temporarily. I wanted to run 14" steel Mustang SSP
look a like wheels off a T-Bird (real 15" SSP's are getting real expensive) but the offset is just too narrow for the Pinto and that too looked dumb.
So, yes it is doable but watch the offset and the fender clearances. Lastly wheel size can become irreverent when say... a 185-50-15" tire may be very close in actual outer size to say... a 195-70-13" tire. It is only the large hole in the center that is really different.