Novak's will modify your SM-420 top cover by drilling and tapping a hole into the tunnel that the reverse shifter shaft slides in and then installing a switch in the hole. Last I checked they would do it for $120 if you supply the tranny top cover. The switch has a ball shaped "button" that the shifter shaft presses on when the rod is in the "reverse" position. I just don't want to tear mine apart.
'57 GMC 102, Original 347 V8, HydraMatic, 3.08 rear gear, added A/C, disk front brakes, HEI, AFB carb, '98 Honda Black Currant paint. T-boned and totaled 10/12
'52 GMC 152 Stake Bed, Original 228, SM420, added A/C, HEI, disk front brakes, '67 Chev 3.55 rear gear. Gets used as a real truck.
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