Jeff is right, the 3 speed was standard equipment on all 1/2 and 3/4 ton trucks. The SM420 4 speed was an RPO.
Also, the 3/4 ton will have a strange hybrid driveshaft set-up. It will have a torque tube partway back, then a u-joint, then the rest of the driveshaft will be open.
-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's
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