Time for new tires and rims for the 68 Dart

They are 5x4.5. To your point, it's time to start calling some manufacturers.

Go to summit racing and set the filter to 15" wheel, the width you want, and the desired backspacing and you'll see more wheels than you will know what to do with.

"Measuring from the face of the drum to the edge of the spring is 6.25"

What is the measurement from the face to the quarter?

What width and backspace are you looking for?