Rear leaf spring Question 74 Dart

Not much... You can check but I think they are 4 1/2 inch wide rims. If so you could do some 205-70-14s.
The easiest thing to do is find a set of factory 14 inch big bolt rallye wheels. They are 6 inches wide and open up a lot more possibilities. You might be able to find a set of used 14x6 inch wide steelies like you have that would take your factory hub caps, but make sure they are straight by taking the bare wheels to a tire shop and have them spin them on a balancer and check for straightness.