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

By the math, a 215/70-15 is about 26.85 tall.
And yes it will bolt on, but you will not be able to steer to full lock as the tire will rub , depending on the Backspace in at least two places, First is the bottom front corner of the fender, and second is possibly on the frame rail.
If you run any non-stock offset wheel, and a taller than stock tire, then the Scrub-radius will cause the tires to pull.
You might have had better luck by using the stock wheels and tires but just raising the ride height.
IIRC the stock tires were about 24.5 tall
Since you are not afraid to change things; and
the wheels are already purchased, and
the T-bars are on the way
I suggest a 225/60-15 at about 25.6ish tall
If required;
The checking rim for a 225 is 6.2. The 7.0 is already at 79% section-width, so that should be fine for a 225/60.
The fenders are easily trimmed,
The ride height can easily be adjusted,
The steering stops can be welded,
However, if the tires are coming from the mainland, I suggest to buy just one, and make sure they fit on both front and rear. Then make a decision. Or maybe you can find a junk tire kicking around your neighborhood.
I bought a Milodon roadrace pan. She has bump-outs sideways but is till a bit below the K-member. I just built a skid-plate, and bolted it on, took me like an afternoon. Now my car can fly! Speed bumps here we come.
Maybe you can build a plate too.