The 57-59 Ford will have a 26 1/16" driver side / 30 1/2" pass- side axles. Add the 1 1/8" gap between axles and 1/4" for drum thickness and you have 57 11/16" total. Pinion approximately centered. 65-66 Mustang will have 26" driver side / 30 1/8" pass- side axles. Also add 1 1/8" gap and 1/4" for drums and you have 57 1/2" total drum to drum. Pinion approximately centered. Hope this helps.