Here ya go Brian. Danged if I know what it is.
but it's not an SM420 which I thought would have been behind a '58, 261:
http://www.pbase.com/dennygraham/oddball_transmissions

Got nothing for '58 transmissions, but if it's the same as
the '59 this might help:
http://chevy.oldcarmanualproject.com/chevyresto/59tsn04.htm
Looks like it could be a New Process or Spicer.
Try to find some numbers on it.
Denny Graham
Sandwich, IL

Last edited by Denny Graham; 08/15/2015 10:39 AM.
Denny G
Sandwich, IL