take a deep breath I have been in this situation more times that I would like to admit in my short years of a home wrencher....
if the engine isn't cranking at all its electrical.If it is but not firing you need to look for fuel/spark.
I think you mentioned no crank so work backwards from the engine. Start with your main ground wires. I am guessing they are on the drivers side frame rail somewhere. Back them out clean and retighten.
Next is starter. I am leaning that you have a faulty starter but make sure all connections are tight and wires aren't exposed. I would run to auto zone and get a new one. Its a quick and easy way to test it - they take returns no problem.
I think your problem lies there - you already did the switch.
Download the wiring diagram here this is my friend Mikes wiring company and the schematic is close to stock:
https://www.americanautowire.com/sho...66-chevy-truck