Truck has good battery but can't get it to turn over?

[White70C20]

Hello all,
I am having another electrical problem with my green '70 C20. It has had an issue with not starting before, but I have always been able to remedy it fairly quickly. Not anymore.

Last Sunday I went out for a drive, I had to jumpstart it (none of my usual remedies worked). I noticed that my aftermarket tachometer was bouncing around at low RPMs and not giving me a correct reading. I also noticed that it was misfiring. When at a stop I noticed I had no lights either. then it died. When I tried to restart it nothing happened, no click, nothing except the tachometer needle would bounce when I turned the key back to OFF.

I got it back to the college and left it alone until today. A friend of mine made a very convincing diagnosis of a sticking starter solenoid. He said that the solenoid sticking could cause a short that could affect the whole electrical system (is he full of BS?) He said to replace the solenoid. It was only $15 so I thought it couldn't do any harm. Tonight I replaced the solenoid and nothing changed. I got everything hooked back up it still wouldn't start. I fixed a worn, frayed battery cable and now I have all my lights back, but it didn't help the starting.

Sorry for the long post. Does anyone have any suggestions? Thanks!