I was fortunate when I bought the overdrive trans for my 58 Suburban. The seller had the entire wiring harness pulled from the donor vehicle (67 C10) and zipped tied ready to go. I got the relay, kick down switch, all the wiring and all the parts for the overdrive (solenoid and governor). I probably paid a little more than I needed to but the seller had driven the vehicle it came from and could vouch for the operation of the overdrive. It was complete and ready for installation.
I wired mine slightly differently than the factory diagram. I have a 78-79 250 with HEI ignition in my truck. I couldn't find any way to attach the kick down switch to the carb. My fab skills are pretty non-existent. I ended up mounting the switch (a simple push button thing) in a spare hole I had in the dash. Also, with the HEI there is no place to hook up the ignition shorting wire like on a points ignition. In order to drop out of overdrive back into direct drive I just have to push the button on the dash by hand and let off the gas for a second so the solenoid can disengage. After a second or so I step on the gas and it runs in direct drive. As soon as you let off the gas again the solenoid fires and it goes back into overdrive. Took a little practice but it works like a champ. This is very handy for highway driving when you need to go up a hill or need a little more power/torque to pass.
The other change I made (based on advice from another Bolter) was to add an on/off switch inline between the kick down switch and the solenoid. When left in the on position the overdrive functions normally and I use the kick down button as described above. When in the off position it kills power to the solenoid so it can't go into overdrive. I found this very handy when cruising local roads and coming into a town or something. Instead of pulling/pushing the cable to disable the overdrive, I can just temporarily do it by flipping the on off switch. When I'm out of town and back on the open road I turn it back on and it works as designed.
The only "interesting" byproduct of my installation is that with pull cable in the on position and the on/off switch in the off position (temporarily deactivating the overdrive) the transmission free wheels at any speed. There is no engine braking at all like there is when in direct drive or overdrive above 30 MPH. It allows me to coast along when not accelerating. Another feature that took a little getting used to but is very handy.
The little clip that holds the end of the pull cable housing to the transmission is a bit wonky and doesn't hold the cable housing very well. After pushing/pulling the cable a few times it tends to slip in the clamp and then I have to crawl under the truck to fully lock in/out the overdrive. That is why I really like the on/off switch. I leave the cable in the overdrive position all the time and just turn it off electrically when I don't need it. If I ever tow something I'll make sure to use the cable to lock out the overdrive. One of my short term goals when I get my truck back is to figure out a better cable clamp so I can really use the cable as intended.
I'm hoping that I'm not somehow endangering my overdrive with the on/off switch. I drove the truck for about a year with that setup and never had an issue.
All that being said, IMHO you really need to have a kick down switch or some other way to disengage the overdrive other than the cable. Right now it sounds like you just drive around in overdrive all the time and don't have any option for direct drive other than pulling/pushing the cable. I guess it will work that way but it really limits the driveability functionality and benefit of having the overdrive. I would be concerned about wear and tare/load on the overdrive planetary gears. It takes a lot of force to move a truck from a dead stop. Much less than dropping into overdrive at 30 MPH. I would think you would be banging pretty hard on those gears every time you take off from a dead stop while in overdrive.
Just a different option for you to consider.
