I went through this on my 67 this past spring. You did not say if the 4-speed manual was a muncie, but I thought I would share this. SS396 and OPGI and I'm sure others sell a reverse lamp switch that mounts next to the reverse lever on the side of the Muncie transmission. There is a switch arm that connects the switch to the reverse gear lever. The switch comes with a wiring harness and a wiring harness extension. Around $100 for the two of them. The extension harness plugs into the reverse circuit that should be next to your Neutral saftey connection at the bottom of the steering column. You have to pull the reverse plug off the steering column (green and pink wire) and plug it into the extension harness. Then you can connect the opposite end of the extension harness into the harness for the reverse switch. This is before you mount it on the Muncie. You should be able to switch your vehicle ignition to the ON position or just start it up. If you rotate the lever on the switch you should be able to test if the reverse lights work. Ok that addresses the wiring.
Now you have to mount the switch to the Muncie. I found this to be a pain in the ass because I had to jack up the car and found that the lever arm that connects from the reverse gear lever to the switch lever was not long enough. I had to go to Tractor Supply and make my own. Send me a private email if you want instructions to do this.
You also have to figure out where you want the harness to run. The extension harness, on a true SS, is suppose to go from the green/pink wire connection through the firewall just above the steering column. At the end of the harness is a rubber grommet wire connector. Please forgive my descriptions, I'm not a mechanic. That rubber grommet attaches to the firewall. Then you connect it from the switch next to the Muncie to the grommet in the firewall. That is the way it was done on Chevelle SS and Camaro SS vehicles.
I did not do it that way. After mounting the switch on the Muncie, I ran the harness up through the shifter tunnel cut out. I know... it was a hack! I then plugged it into the extension harness and plugged the extension into the shift column Green/Pink wire plug. I used some zip ties to secure it keeping most of the slack of the harness inside the vehicle. I may not keep it that way and may go through the firewall as I indicated above.
For me the harness and extension harness worked perfectly. My only problem was getting the switch to work properly on the side of the Muncie. The biggest problem was the connecting rod that was sent in the packaging. It was cheap pot metal. I tried to bend it slightly to get more length out of it and it finally snapped like a twig. I was off to Tractor supply and picked up a similar diameter rod. Did some measurements and a couple of bends and its now working.
Oh BTW... the connecting rod has a groove for some securing pins to be placed which prevents the rod from falling out of the hole on the reverse gear lever and switch lever. These pins are VERY small!!! Easy to drop and loose. Be careful. I had to delicately use a cutting tool and recreate those grooves in the connecting rod I had to make.
Best of luck. Took me about 4 hours to get this working correctly. But then again... I'm a newbie at this.