front backspacing

The math puts the diameters of those two nearly identical, at 25.6inches.
-The 14"rallys were,IIRC, a 5.5 wheel with a 6.5" max outer width. IDK the backspace on those, but might assume about 3.25 for a Zero offset.In which case the outer edge of the rim would be 3.25 outboard of the mounting flange.
-Your new wheels are 15 x 7, which is 8" max outer width. You specify a 3.75bs. This puts the outer edge at 4.25 outboard of the mounting flange.
- Meaning........The new wheels will push the tread 1 inch further into the fender, at the straight-ahead position, and will push the treads out 1 inch closer to the front lower corners of the fenders, when cornering.
-A 6 inch wheel with 3.75 bs, being 7'' max outer width, puts the outer edge at 3.25 outboard of the mounting flange, same as my assumed rally wheel.