Can't start classic Grand Prix (long question, sorry)

I have a 1969 Grand Prix that had been running fine. During winter storage this year I decided to work on a few issues. 1st I removed the dash to resolve some accessory problems. The dash went back on and I started the car fine. Then…I decided to convert the ignition to Pertronix and eliminate points since I couldn’t get the dwell accurately. I also removed the linkage from the automatic floor shift to the steering column because it looked a little rusty and cleaned it. (I did reinstall it) When I tried to start the car I get nothing. No starter spin, no noise at all. The battery is fine. I tested it and even tried another new one. I took the Pertronix out and put the points back in, nothing. I replaced the coil with the new Pertronix, nothing. I thought maybe it was the neutral safety switch, which is located on the steering column and activated by the transmission linkage that I had removed, but I can see the linkage rod moving the switch. I even tried a new neutral safety switch. I replaced the ignition switch and that didn’t help. (since both those switches were original I didn’t feel bad about replaceing them even if they were not the problem). I don’t find any bad fuses. The headlights and dome lights work fine. I’m thinking it’s something simple but don’t know what else to check. I haven’t tested the starter yet. Is there an easy way without pulling it off? Any suggestions as to what it could be?

The first thing that I would check is the Neutral Safety Switch.
It is possible that you did not reconnect it when you reassembled the shift linkage.
It is even possible that the NSS was damaged during removal and reinstallation of the linkage.

The first thing I did check was the NSS. I took it off and checked to see if it was sliding/operating correctly. The rod linkage engages the switch and moves it from side to side. I also had a brand new switch that I put on it and tried. I took the switch back off the column and just left it hanging and manually moved the mechanism while turning the key and still got no response. So, I don’t think it’s the NSS.

Try changing the negative battery cable. Check for cracked rubber on it. I checked one on a 73 Maverick by connecting both jumper cables to the metal on the engine and the other ends to the battery end of the cable. If your car has side terminals, you may not be able to do it. When the engine started, I knew what the problem was.