Bill is correct. In 1964 the standard gear in the heavy Galaxie with an automatic was a 300. Four speeds had 3:54. With your light car, anything lower than a 3:54 would be a waste in my opinion. I think around the 300 area would work great. And many of the old Lincons did have a posi unit, but if you don't have one you can still put one in.
Ron
