Ford shop manual page 11-13
1968 Mustang
with 200 inch 6 cylinder:
4 blade fan - 15" diameter, with standard cooling and No air conditioning.
6 blade fan - 15" diameter, with extra cooling or with air conditioning.
with 289 or 302 engines:
4 blade fan - 17.5" diameter, with standard cooling (20 inch radiators) or extra cooling (24 inch radiators). No air conditioning.
5 blade flex fan with air conditioning - Must have 24 inch radiator.