There were TWO types of thermostatic fan clutches. The earlier ones, from the earlier '60s which were rpm-limited, with no thermostatic spring on the front cover. Later, the thermostatic-type with the spring on the front to modulate when the fan operates or free-wheels. By '65, they should have all been the spring type with a/c.
Mopar Perf used to sell and OEM-spec fan clutch and fan kit. Would not fit the '63-4 cars due to it having the '65+ shaft length. The fan was a more modern 7-blade, as I recall. Pricing was very "right", too! Not resto-correct for all cars, but was a good deal.
As these OEM-type thermostatic fan clutches depend upon the temp of the air coming through the radiator "to work", if the radiator becomes clogged internally and the air through it is more ambient rather than engine temp, the thermostatic spring will not let the fan clutch engage (as the temp needle rises). So a "clean" radiator is important, inside and out.
CBODY67