Power booster again

What is the difference between the 65 power booster and the 66? I cannot find a local source for a 65 power booster but have found one guy who said he thought he could get me a 66. If I buy a power booster from a mustang place it runs $300+, if I can find it locally it will run around $100. Everyone that I have talked to is scared to order one because they can't guarantee it will be the right one. Where can I find the right part number, any suggestions.