The part numbers listed for the belts are:
Alternator without the thermactor (T/E) pump, without AC - CQPZ 8820& (JB-412} 15/32 " x 41 1/4"
Power steering without the T/E pump, without AC - either 0CQPZ 8620-CB (JC-460) 1/2" x 46" or CC2TZ 8620-G 1/2"x 45 1/2 ". The first is used in the US only, the second in Canada only.
I find it best to remove the belts from the car, take them into the local auto parts store and ask them to match them. If you are certain that your car accessory drives and pulleys are all original, you can also order replacements from a classic Mustang parts house like National Parts Depot.