Interesting trivia I had not seen previously.
If you search AutoZone for 'radio capacitor' it says there are no parts by that name. However, you also get a 'Points and condenser' link which brings you to two 'radio capacitors' which AZ sells (while saying they don't??). Both are made by Wells Vehicle Electronics which I also never heard of. Seems they built the Norden Bomb Sights in WWII; that business has been slack for a few decades.
Going to Wells website its also hard to find these but under 'Specialty Applications/Capacitors' you get 4 choices which are all 'radio capacitors' which I didn't know anyone still made. Two of them are sold by AZ but this verifies that the AZ listed lead lengths in FEET should really read 'inches'. Only the URS1500 has any electrical data listed: 0.5 MFD, shorthand for 0.5 microfarad. The app note says: Universal type for noise elimination for generator brush arcing. The original Ford caps were also ~0.5 microfarads so that isn't electrically far from an original. Their own words though hint that its not much good on a alternator whose 'noise' is more a single frequency sine wave whine that a single capacitor won't help much.
Knowing that a regular ignition condenser is the same item except with half the capacitance (~0.25 microfard) the $15.99 is also kinda pricey especially without the Ford bullet connector on the end.
The technical description at AZ is pure techo-babble written by someone who hasn't a clue what these are or do. They have little to do with the (ignition) coil current; the noise generated from the coil is mostly RF noise that goes through the air and is unaffected by a radio condenser on the 12V power. You put resistors in the spark plug wires for that kind of noise. Some cars did have caps at the coil for noise suppression on the input power but Ford never put one there on a Mustang. And to suggest that not replacing this part will somehow cause "no spark and/or no-start" is pure and simple fantasy.
Seems these have been around awhile. This one looks fairly 'vintage': http://www.ebay.com/itm/Wells-URS1500-Condenser-/110808885114