[Fordors]

Info I have from Hot Rod Magazine Feb. '68 shows this:

Perches C to C

'20-'27 48"
'28-'31 49"
'32-'34 47 1/2"
'35-'40 48"
'41 46 1/2"
'42-'48 47"

Rear Spring C to C
45"
46"
43 1/2"
44"
41 1/2"
43"

BTW, he's right about the angle on those shackles. There is no way they are working as intended; your spring is too long if you do measure your perches at 46 1/2"