The 67 F250 and your Mustang have the same blower motor.
First, to be sure you understand how the electrical hookup works on that motor. The motor is hotwired so it has 12 volts to it at all times. The three speed switch directs current to one of two resistors mounted to the side of the heater box. It goes to the biggest resistor for low speed, then the smaller one for middle speed, then bypasses both for high speed. Easy check on the motor is to turn on the ignition and then ground the wire going from the resistors to the blower motor. See if doing that makes it spin any faster.
It sounds like the existing blower motor needs the wiring fixed or needs to be lubricated or needs to be replaced with a new motor. Check the wiring connection in the engine compartment where they feed into the motor housing. Usually they are brittle and maybe have one or two strands of wire remaining. That definitley will affect performance. Another problem could be if the resistor set is not working correctly, keeping the motor from being properly grounded. The original 66 heater motor should blow more hot air than you can stand to be around.
I would pull the heater box, remove the motor and see what is going on with it. You also might find a control door in the box is not opening or closing properly, which could be blocking flow.
If the motor feels like it spins slow by hand, with some bearing drag, then it is time to lubricate it. The end of the shaft near the blower cage is easy to get oil to. Just stand the motor vertically and put a few drops of oil on the shaft while rotating it until the bushing gets lubed. It may take several hours to accomplish this. Put some oil on it, let it sit, rotate it, then repeat the process until it gets lubricated.
The other end of the motor is a different story, since it is a sealed assembly. You will need to drill a 1/16" hole on the lower side of the motor, right hear the end of the motor housing. I say lower side because when you get through it won't show that way. Once you get oil in there in similar fashion as the other end, clean it really good and then slightly dimple the hole inward. Then put a dab of RTV on it to seal it. If the oil makes it where it spins easily, then you are ready to reassemble.
Get a new foam seal kit for the heater box. Clean the old stuff off and use upholstery/carpet adhesive spray to attach the new seals to the interior doors. When you reassemble it, it will work as good as new. This is an opportunity to install a new heater core if you think you might need one in the near future.
Luckily the entire heater assembly can be removed in a 66 in about 15 minutes, so it is not a tough job.
When you reassemble it into the car, attach the heater hoses to the heater core before placing the assembly in the car. Be sure clamp fasteners are facing down so you can access them in the future should you need to. Push the hoses through the firewall as you insert the heater box and have a helper hold the hoses to align the box while you put the one vertical underdash bolt in place, to the right of the glove box. Once that bolt is in, then go put the four on that hold the motor to the firewall. I would advise oiling the control cables while you have it apart for like new function when you get it all back in.
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