If you have the big bolt pattern then I would go with a 15x7 with around 4-4.5 back spacing on the back and a 14x6 in the front. If you have a small bolt pattern it might be harder to find wheels but the size I mentioned will still be good.
I think I would try to find some 5.5 wide rims then you could run a number of sizes. I personally like the 235/60/14's and had those on a set of small bolt rallyes on a 66 cuda and it looked pretty nice!! I dont think you are going to like the 4.5" wide rims but if thats all you have to use...
Years ago I purchased a set of 15" ford magnums that were off a friends wrecked mustang and the center hole was smaller and wouldnt fit over the hub center of my Challengers disc brakes. That would be what I would look out for when buying the ford wheels.
Look on the rim lip by the valve stem and there will be a part number that you can reference as to what they came on. As far as them fitting on your Dart, I run 15x6.5 with 245/60x15 tires on the rear of my 69 with no problems.
I run 245/60x15 on 15x6.5" rallye's on the back and 245/60x14 on 14x6' on the front with no issues on my 69 dart. I have super stock 002/003 springs in the stock location.