The SM465 is the replacement for the SM420. It's first gear isn't
as low, but it has a sturdier case and uses a larger input bearing.
Input bearing retainer size may also be larger though, so finding a
bellhousing for anything other than a Chevy SB is not possible from
the junk yard. It is also a bit longer than an SM420 which may be a
concern for CJ-5 and earlier Jeeps.
SM465 side view
The SM465 has been used in Chevy trucks since the late 60s. The
following articles cover some things to look for to find the SM465 you
GM SM465: Chevy used the SM465 from the late 60s until the 80s.