just as wayne stated, there is a 1/2" perch difference... but all 57-62
full size [exc imperial] accept all C-body rears, '65-73, as the 97% bolt-in
rear. the hydraulic line mounting point [rear vent] is off by about 6", i
weld a 3/8x1.25" bolt in the location of the original 57-9 position, to locate
the junction in the proper spot, thus enabling the transfer of both rubber and
solid brake lines to the new rear, which even accepts the stock driveshaft, exc
for early 57's with bolted u-caps... mid 57-up use the modern-esque strap and
fine thread 1/4 bolts. when relocating the hydraulic junction, always
replace the breather in the modern position closer to the center section.
as for the perch difference, i swung in a 67 rear in my 59 fury, which has
been a daily driver since about sept '01, and about 45xxx miles later, the
springs actually negotiated themselves slightly outbouard on their
bushings... which was probably aided by about 25 or more full figure 8's,
countless 200' plus suregrip burnouts, 4 tires-smokin-till-they-pop burnout
contests, and many winters getting through the snow better than 4x4 suv's with
my recap radial snow tires [with portawalls :) ]