If it's anything like my 69 Camaro, and you're not getting power at the fuse block, I doubt it will be the headlamp switch. The power hits the fuseblock first then the lamp switch.

Check the connection on the engine side of the firewall at the fuseblock. You might want to disconnect both the harnesses and reconnect them. Also, while you've got it disconnected you might check the continuity of the pins and the positive lead from your battery. If there isn't continuity at the appropriate pins the problem lies between there and the battery. If it's good, start looking further up the circuit.

I had some terrible problems with the wiring on my Camaro. Besides some previous owner doing all sorts of hacked custom wiring (all in black too) in the engine compartment there were all sorts of intermittent problems at the fuseblock connection. I really, really wish I replaced the engine, headlamp and tail lamp harnesses much sooner than I finally did.