T_Doug,
you need to look for gear sets for a 4 speed sm420 tranmission,
GM listed a different part numbers for the same ratios between the two transmissions suggesting they couldn't just use the same part between 3 speeds and 4 speeds.
non of the gear sets for the 3 speed overlap to 4 speed sm420 applications in the parts manual that i could find
that suggests they physically wouldn't fit
-s
