Your assumption is correct.

Key activated (solenoid) starters were available in '57 six cylinder trucks as an option. My GUESS is that any 12 volt key activated starter from a 235" six cylinder Chevy would do the job. Ignition switches with a "start" position, key cylinders and mounting bezels are available at the "usual suspects". Your already there bypass resistor should be used.

'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.