We always recommend the 215/60 x 15 tire, particularly for 65-66 models that do not have as much room as the later years. The yellow Mustang pictured is lowered a bit, but has basically stock suspension, and this size works great.
We just got a huge shipment of these wheels in, so they are in stock and ready to go. You may want to just order our wheel and tire package - that's 4 wheels with centers and 20 lug nuts mounted on BF Goodrich Radial TA raised white letter tires. They come to you mounted and balanced. Just bolt them on. It cannot get any easier than that.