If it's a '67 C/10, the 4-speed should be an SM420 and if it's '68 or later it should be an SM465. Not counting any PO's modifications or if it was made in Canada or by GMC. [They kept the 420 for a model year or 2 later.]
I/I #4101
'71 GMC Jimmy 350, sm465, np205,3.73 posi.
'68 C/10 Stepside 292 (.030 over) Offy Intake, 500 CFM AFB,Clif headers, sm465, 3.73 posi.
'67 K/10 454 project.
'72 K/5 rolling frame project.
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