that makes sense. What threw me off is this quote from the Kevinstang site. Reading it, it is easy to get the impression that he is talking about spring perch distance here on different year Mustangs, but what he is really taking about is Fairlane vs. Mustang. It stands to reason that if the housing is 57.25 or 59.25 flange to flange there's no way the spring perches would be 52 to 54 inches apart. Common sense, I guess.
"I can tell you for a fact, the distance between spring perches is different between Mustangs and Fairlanes.Spring perches must be cut and re-welded inorder for the swap to be performed. The “rough" widths I keep in mind for Mustangs are the following: 52” for 1965 to 1966 (the same width as 64-65 Falcons and 62 to 65 Fairlanes)(as in the Mustang line, most however never received a factory 9 inch), 54” for 1967 to 1970 (same as 66 to 69 Fairlane,Torino,Comet & Cyclone non station wagons)*cars with 351 and up engines received 9 inch units-as did some 302 4V cars-(mainly GT's in 68 with optional gear ratios)* and 56” for the 1971 to 1973 Mustangs/Cougars- (cars with 351 engines and up receiving the 9” housings).Keep in mind as mentioned, the Fairlane spring perch distances were not the same as the Mustang. All the Galaxies I have seen from throughout the 1960’s used the 9 inch rear, regardless of engine size. "