Correct.Half tons are 5x4.5" until 85 or 86 I think, and them go to 5x5"
'65-67 differential housing is almost a perfect fit for FMJ.
59.500" drum to drum
54.250” flange to flange
44.000” center of perch to center of perch
The differential is 1" wider (1/2" per side) and perches are 0.230" narrower (per side - which is almost negligible).
The big issue is those '65 axles. You have to get rid of those axles - for doing brake work on a normal basis is almost impossible (as well as finding brake drums). Getting '66-67 axle shafts is somewhat easy to find.
That said, I would love to get my hands on that differential - if you are not.