1976, small bolt pattern. original axle (our family has owned the car 33 years, 2nd owner, it turned over 50000 miles on the way home from buying it from the original owner, who was 83 in 1991)
I believe you, I just wanted to confirm some of the details. Because the stock BBP 7.25's are absolutely not that width, the housings are narrower and the WMS-WMS is substantially narrower than even a SBP 8 3/4. Your measurement is the same as a SBP 8 3/4, but that would require a totally different housing width than the BBP 7.25's.
The BBP 7.25's are well documented at a WMS-WMS of 56-3/4", and a housing width of only 51.5". I had one myself and confirmed, and you can look up any number of wheel/tire threads with people fitting wheels that would fit an 8 3/4 with BBP axles not being able to run the same combination on a BBP 7.25 or 8.25 (same width as the BBP 7.25's).
I do not have good measurements for a SBP 7.25 of any year, haven't ever had one myself and most people don't bother to run big wheel/tire combos on them. Based on the wheel fits I have seen I suspected they were probably closer to SBP 8 3/4 width than the BBP 7.25's, but with body tolerances such as they are I wasn't 100%.