The 'real' (not reproduction) 14 inch Magnums came on 66 Shelby Hertz cars and 1971 Torino's. 1972 Torinos and Rancheros were offered with them too. Other similar wheels that used trim rings were available on mid-70's Rancheros, -possibly others. No 14 inch Magnums were factory correct for 71-3 Mustangs.
From the 'net:
Most common Shelby mixup - original 14x6 1966 Hertz Magnum vs 14x7 1971 Torino Magnum (D1OZ p/n). Look closely at the area where the center meets the outer rim. The difference is the depth of the ridge at the base of the rim, which is the offset for disc brake caliper clearance.
The D1OZ 14x7 was sold by Ford in service for many years as an interchange part, and was also commonly available from Mustang and Shelby parts vendors, so most '66 350's restored in 1970's - 1980's received these wheels.
The third example is a 14x6 for a non disc brake application. Note lack of a disc brake offset. Commonly found on Chyslers of the period, but also sold in the aftermarket by Motor Wheel Corp (Manufacturer of the Magnum 500). Mount these on your GT350 and the front wheels will not turn!
Ray