15 x 7s, what size would you run on a 68 four door dart ?

Those are likely going to be too tall and little too wide on the 7" cop rim for the front of that car. Depending on the offset of the wheels you may also have problem in the back.

There are a lot of threads you should take the time to search on this site that cover “what size tires"? Or "how big of a tire can I go" etc….

Good luck