Tony's got the scoop - and to make it easy if ya want it to look original, the repo dealers carry the correct wiring harness for 55-65 cars and trucks, like Carter for one - EL-151, $22.05 - I got one from GoldenState years ago when they were cheaper, like about 12.95

also got a solenoid and relay back before they disappeared

and kickdown switches can be adapted from a whole bunch of modern apps
does sound like this guy has just put together info and parts available to anyone, and is makin a wage for watchin ebay and shipping stuff - and buzzin, the copyright belongs to GM!
Bill