You did not state the year. For example, in 67 the use of 15" wheels was with the ultra rare competition handling package. According to the 1967 version master parts catalog, the wheel used for that was C7ZZ-1007-D. The specs are 15" x 6" x 5/8" offset - 5 bolt holes - 4-1/2" bolt circle.
Looking at other 1967 model year cars;
1. "A" body full size Ford (427/428 and station wagon, conventional brakes) used C5AZ-1007-B, 15" x 6" (no offset listed) - 5 bolt holes - 4-1/2" bolt circle
2. Full size Ford (except 427/428/station wagon/police interceptor, conventional brakes) used C5AZ-1007-A, 15" x 5" (no offset listed) - 5 bolt holes - 4-1/2" bolt circle
3. Full size Ford (taxicab with disc brakes) used C5SZ-1007-A. 15" x 5.5" (no offset listed) - 5 bolt holes - 4-1/2" bolt circle
4. Full size Ford (disc brakes) used C6AZ-1007-A, 15" x 6" (no offset listed) - 5 bolt holes - 4-1/2" bolt circle
5. All "B" bodies (Fairlane) used 14" wheels.
6. All "F" bodies (Mustang) used 14" wheels except as mentioned above
7. The "U" body (Bronco) used a 15" wheel but there was no offset and the bolt circle was 5-1/2". Also had a 16" wheel.
8. "X" body (Falcon) used 13" and 14" wheels.
According to the 65-72 conglomerated MPC, looking strictly at the "F" lineup, there are the following 15" wheels:
1. 1965 - C1AZ-1007-A, 15" x 5.5" - 5 bolt holes - 4-1/2" bolt circle. Note that there is no offset listed with ANY wheel in this version of the MPC.
2. 1966 same C1AZ-1007-A listed
3. 1967 - The aforementioned C7ZZ-1007-D is mentioned, this time specifying "except GT350/500, competition". No offset is listed like it was in the 1967 version of the MPC.
4. 1967 - GT350/500 lists a C8AZ-1007-B, painted black, 15" x 6" - 5 bolt holes - 4-1/2" bolt circle. There is a note that it is replaced by D0LY-1007-G
5. 1967 - GT350/500 also lists the Magstar 15" x 7" wheel option.
6. 1968 - only listing is C8AZ-1007-B, 15" x 6" - 5 bolt holes - 4-1/2" bolt circle. There is a note that it is "stamped" C8AA-1007-DW
More options starting with 69 for Mustangs.
When I look through the list, considering that I "trust" the 67 year specific MPC more than the 65-72 version, although admitting that I have found errors in it, it looks like there are a plethora of choices for Ford 15" steel wheels. Knowing that all manufacturers love to "carry over" parts to avoid the cost associated with design, tooling, and handling, since there are so many unique part numbers involved I would conclude that slapping any old 1960's Ford steel wheel that has the 5 bolt, 4-1/2" pattern would be somewhat of a crap shoot. There must be something different in order to have created a new part number. Since the diameter, width, and bolt holes are clearly listed, the differences are somewhere else. Back spacing is the most likely candidate.
The backspace listed in the 67 MPC and not repeated on the 65-72 version is a bit disconcerting. I suppose though that if you went to the dealer to buy wheels for a year "X" Mustang, they would only need to be concerned about the diameter and width. At that point all part numbers identified as fitting the Mustang will have the correct back space, so there is no need to specify it. What is interesting is that the 15" wheel for the 1967 Mustang is the only one that mentioned backspace. All others were not specified.
I do know that at the swap meets over here in the US that I attend, "everyone" is asking for 15" wheels. They are not plentiful, and when found go quickly. Even looking through eBay they don't come up often.