Rotary cutter sizes are like buying lumber at the local hardware store. The size isn't exactly as advertised. The average 60" cutter will actually cut around 55" or so well.....likewise up, down, left, and right.
As for the blade thing..........visit the KK or Howse website. These are the two leading "economy" sellers of 3pt implements today. You can deal directly with them each on everything from buying complete implements down to basic parts. And I have with success on more than one occasion with Howse. Good experience and I'd recommend dealing with either directly as opposed to waiting on TSC or the local retailer to get things done. As for FF/Tartar? I don't see the option to deal with them directly on their site. Seeing as how bearings, blades, bolts, and fittings today are more universal........you might check and see if the KK blades will not just fit right on your FF rig with little/no trouble and get on with things.