No help on a source for the best radial tires but because I thought your comment of only two having radial tires seemed VERY low, I went to Kevin Marti's Mustang By The Numbers book.
1,383 in total 1967 Mustangs came from the factory with Radial Tires. No size is mentioned in his book.
Breakdown by body type is 363 Coupes, 855 Fastbacks and 165 Convertibles.
Numbers on a Marti Report are typically arranged in an order to filter YOUR particular car towards a goal of "1 of 1", juggling the numbers of OTHER options to reach that goal.
Interesting. I did not know they had a P-Metric tire as an option in 1967.
The "P" in "P-metric" statement did not come into play for about another 10 years (1978?).
Notice the O.P. nor Marti Reports have the letter "P" in the tire size.