So, the truck (1967 C20 w/350 V8) has been running great. Tonight I pulled up to a stop sign with my high beams on, and as soon as I put on my right blinker, everything dies. Electrical everything. Blinkers, starter, lights, absolutely everything. Fuses all look good (although its about 3A, but with a flashlight they all look operational). Jump start yields no results. No response to anything. I checked all the major connections, grounds, and fuses with a voltmeter with the key On and Off. Any prior experience, anything I couldv'e missed, or ideas?

Check the positive cable going to the starter, I've had mine dead short the battery when it got up against the exhaust pipe. Check with a test light or multimeter to see if you're getting juice to the little junction on the fender by the battery, if so, check the fusible link in the wire going from it to the fuse box.

I don't know if this will help but here it is.

that's how they did cars, but not the trucks.
Case in point, you can completely remove the horn relay from your truck and still have power.

first place i'd check would be the 2 battery terminal connections// both ends of both wires