There is nothing factory that you can bolt onto your current drum spindles.
Either of the factory options will require a spindle swap. Your December '72 built '73 should have the later, 73+ large ball joint upper control arms, the swap over to the large upper ball joint UCA's was finished by November '72 according to the factory TSB. That means that the earlier Kelsey Hayes disk spindles (that use a 5x4" bolt pattern rotor) will not fit your current UCA's. The later 73+ disk spindles will fit, but the 73+ disks use a 5x4.5" bolt pattern.
DoctorDiff now sells a 73+ disk brake kit that uses the 73+ spindles (which will fit your UCA's), but has an option for the 5x4" bolt pattern available for the rotor. That's probably your best solution unless you want to convert the whole car to 5x4.5". @DoctorDiff is very knowledgeable about these swaps and should be able to answer any questions you have.
10.95" Front Disc Brake Kit (Stage 1)
