I too looked and didn't see the resistor available where one might regularly find them. The resistors are simple wire wound type. The Pinto has three speeds on the blower. Two of the speeds pass through the resistors and the full speed is typically directly wired. The switch determines how the power is routed.
Are you sure it is the resistors that are bad? The switch and the wiring could also be the problem. Sometimes the plastic socket melts. I'll offer the following "
Do at your own risk" considerations:
1. Check to confirm that the resistors are bad. Sometimes the contacts are just dirty on the connecton plug or at the switch. Cleaning them might remedy the problem. Sometimes the wires get fried. I have replaced wires and used individual spade connectors in place of the "socket". Just makes sure you note the wire color position before you remove anything.
2. Look to see if another resistor block can be adapted. As you asked perhaps a Mustang resistor set will fit (
). In the end it is just a simple mount that goes into an opening.
As to the cable the "style" is very common. Sometimes I have cut a "too long" cable sheath to the proper length and then inserted the inner cable wire and bent it as needed.
Unfortunately there are no "Pinto" vendors though some people do sell Pinto parts (NOS or used). Thankfully a blower motor is rather simple and it isn't too difficult to come up with a work-around. When we do Turbo motor installs the factory firewall mounted blower motor is too long. In my case I used a stubby MG blower motor run in reverse polarity and all is good. We do what we have to do working with old cars. Especially cars that many deemed "Throw-away Appliances."
You also might find this helpful:
http://www.resistorguide.com/applications/blower-resistor/