what are good tires for 64 dart slant 6

This is a great question.
Your car probably had 13" diameter tires and today the selection is almost non-existant.
So as mentioned you'll probably need to buy some 14" wheels, they came in 4.5" wide & 5.5" wide - a set of the small bolt pattern Rally wheels is a good idea as you won't need new wheel covers.
Assuming you have the 14" wheels a good value is the Kuhmo Solas KR21 205-70-r14.
The 14" and new radial tires should make a noticeable improvement in ride.