A 15x8 wheel with 4.5" backspace on a 1969 will rub while using P235/60/R15. I would go with P235/60/R15 on stock 15x7 Rally's up front. You can use P215/65/R15 as well. That size was OE on 1982-1987 Z28's & 1985-1990 IROC Z's with the 5.0L engine and can be readily found as well.
***1) The overall diameter of the P235/60/R15 is 26.1 inches
***2) The OD of a P225/60/R15 is 25.62 and that size can be commonly found on late 1980's & early 1990's 5.0L Mustang's.
*** 3) The OD of a P215/65/R15 is 26.0 inches
Circumference: 81.68 in 2074.6 mm for P215/65/R15
Circumference: 80.48 in 2044.1 mm for P225/60R15
Circumference: 81.99 in 2082.5 mm for P235/60/R15
In my experience you can fit more tire on the front of a 1969 than a 1967 or 1968 and you can fit more tire on the back of a 1967 & 1968 than a 1969.
In short: the 15x8 wheel is too wide for the front of your 1969. Go with a 15x7 Rally Wheel with roughly 4.25" back space (no more than 4.50") and the P235/60/R15 will work just fine. That's what I have on my 1969. Car isn't here now for me to double check back space....
Good Luck & Happy Motoring!!,