Newbie here and will get around to introducing myself and LC later. I noticed in the video when the seatbelt light comes on that the voltmeter gauge seems to jump up to what would be the normal output range. Hopefully I am posting this correctly since this is my first time.The seatbelt warning light is powered by wiring coming off the ignition switch. Re check the white wire coming off the fusible links going to the alternator. check the fusible links again, and check the wires from alternator to ignition switch.
Note that power to the seatbelt warning light and relay would be downstream of the igniton and would have no affect on the ignition switch or motor but is an indication that the ignition is not getting the 12v it needs. Look at the schematics on 84-90 body and chassis fsm at the back pg 715, 717.
Also the hiccups caused by an engine wanting to stall (ie could be anything or not related to electrical) could be sending little to no voltage to the ignition on a momentary basis causing the light to flicker.
The fact that the motor isn't idling could be a vacuum leak or lack of fuel. Listen for vacuum leaks around the carb. Spray carb cleaner around the base of the carb and around the intake where it connects to the cylinder head (do this before things get too warm). Make sure the fuel is 1/2 in the carb window. do this by looking with a flash light and shaking the truck to see the fuel move. Take off the top of the air cleaner and look down the carb and actuate the linkage on the back of the carb by hand to insure that a good squirt of fuel is going down the venturi.
Check for spark at each plug wire. Do this by removing the plug wire and put a screwdriver into the end of it. (should be many youtube videos on this) Turn the motor (but dont start it) and watch for spark to jump to ground on top of the motor . Hold it about 1/8th and inch from ground .. do this with low light conditions so you can see spark. obviously do not touch the metal part of the screwdriver while doing this and where leather gloves for safety.