I used Dallas Mustang and was extremely pleased with their work, but kind of pricey. Have you considered maybe buying OE replacement belts on ehay? There are several seller who specialize in year correct belts for vintage mustangs at reasonable prices. Just a thought.......
I have used Dallas Mustang for years to do my seatbelts, and my son "tested" them when he totalled the '66 slamming into a bridge abutment. They worked.
Also to consider is the core charge. Depending on the condition and color of the originals, it MAY bring more money to ebay them, and save the shipping cost of sending in the cores, and just buy them outright. If you send in 4 seats, not all of them may qualify as Firsts for a full core refund.
They also have a seconds program for belts that have scratches in the plastic or pitted chrome underneath, that are much cheaper.
I always use Ssnake-Oyl. They usually do about 5 to 10 sets a year for me. I always get them to rechrome the latches or anything that needs it, and they restore any plastic pieces. They always come back looking brand new. They will send you a written quote before they start any work. A set of two lap belts should cost you around 250-350 for everything depending on hardware condition. I just got back a set of four from a 1970 Mach 1 which included the shoulder belts. I believe I paid around 700 for them. I wouldn't go anywhere else.