FWIW

[JOE FULTON <piper_pa20@xxxxxxxxxxx>]

Sometimes you have to see for yourself.

I have three rear differentials in my yard right now
and here are the measurements:

69 Mustang 9-in.

52 inches flange to flange
43 3/8 inches between c/lines of spring perches

68/69 Javelin/AMX AMC 20

51 inches flange to flange
43 1/2 inches between c/lines of spring perches

80 Concord AMC 15

52 inches flange to flange
43 1/2 inches between c/lines of spring perches (which
look different than the AMC 20 spring perches)

That AMC 20 measurement could be in error. Note: all
of the measurements are somewhat approximate because
they were obtained by measuring across the hump.  I
didn't build a square frame for the measurements. 
They are close enough though to show that a late
sixties 9-in rear is a direct replacement for a 68/69
Javelin or Concord/Hornet/Gremlin family car.

Joe Fulton

Salinas, CA