Its not so much the size of the wheel diameter that counts.  Changing from 14" to 15" wheels, you will still want a tire with a similar outside diameter.  Basically you are installing a similar tire with a larger hole in the center.  You can go with a wider cross-section and aspect ratio with the same tire OD.

When you change wheels, be sure to get the correct offset to fit the car.  Let us help you if you are not familiar with that criteria.  Typically a 15x7 wheel is used with a 4" to 4-3/8" backspacing.  Before you consider putting alot of money into 4 lug wheels for that car, also consider swapping to 5 lug larger brakes and rear axle from a V8 car.  The parts are much more durable and larger brakes are pretty much self-explanatory.    The 6 cylinder cars had tiny brakes.