Approximately a P195-75R14. The tires of the 1960's and 1970's varied in size even more than those of today from one brand to another but basically you are talking about a skinny tire. It would have been 3% taller than a 75 series tire and 3% skinnier too.
E series= approximately 195-205
F series = approximately 205-215
G series = approximately 215-225
The 78 is an aspect ratio. A 78 series tire is 78 percent as high as the tread width. A 75 series tire is 75 percent as tall as the tread width. So for example:
P205 70 X 14 = 205MM wide tread section
205 X .70 = 143.5 mm tread height
14" rim plus 143.5mm plus 143.5mm = Too hard for me to calculate, if you want to look up the conversion factor it will equal the height (diameter) of the tire.
Like I said, the letter series fall in the middle of the metric tire designators used today and there is really nothing exactly the proper size made either now or then. If you have an original tire to compare you can match the height.