hinge- there should be an access panel on the inside of the kickpanel just inside the door. you can possibly get a not on the back side from there. you can weld a stiff wire onto the nut if you cant get your fingers in there and maybe use that to position it. jb weld will harden up and then you won't be able to move the nut for hinge adjustment. originally the nut was a cage nut, a square nut that is held in place by a sheet metalcage around it. they are available from the fastener supply places so you could repair it back to a useable stock state. if the hole you have in the sheet metal is square, you could simply buy the cage nut with the sheet metal cage that snaps into that square hole.
https://questmanufacturing.net/produ...-silver-50-pk/
bumper bullet mounting nuts-for siezed and broken off bolts try heating the nut to red hot and let it sit to cool off to room temp. the hit it with some penetrating oil. if there is any of the broken off bolt sticking out file or grind it flat and center punch a mark in the middle. drill into that with a small bit. then drill larger until it will fit an easy out. you can try to remove it now with the easy out but if it is still too tight remove the easy out and heat the nut till red hot, quickly install the easy out and try removing the bolt again. usually works. if it doesn't get the easy out out of the hole before it gets too hot and loses it's temper. try the heat and easy out thing again. worse case scenario just drill the hole out and tap it or cut the nut off and weld a new one on, or drill right through the old nut with a bigger bit so the bolr slides right through and weld a new nut onto the end of the old one so the old nut will act as a spacer
coolant reservoir- that spot should work fine. ensure you have a rad cap and rad filler neck that match for a closed system and good seal on the hose that connects the two. also the correct pressure rad cap, likely 15-16 lbs. if using a stock old truck rad also ensure it will take that kinda pressure
horn wiring- the horn button simply grounds the horn relay so it will turn on and honk the horn. I think the horn wire from the column is black or maybe dark green. that is from memory so don't take it for gospel. you could easily test to see which one is a ground when you hit the horn button on the steering wheel.
here is a thread on horn stuff that may help
http://67-72chevytrucks.com/vboard/s...d.php?t=787400
seat belt hole drilling-try a step drill, they drill nice round holes but also stop you from busting through and drilling into something behind your hole, as long as you take it easy.
something to think about would be reinforcing the sheet metal behind the seat belt mount so it will actually hold your body weight should an accident happen, you don't want the bolt to pull through. ensure to use adaquate thickness steel under the nut on the back side
are you planning on lap belts or shoulder harness style?
if drilling through a muli-layered floor area, like where the old seat used to bolt on, ensure to use a tube inside the air space between layers so when you tighten down the fasteners it doesn't just crush the layers down. use a good sized wall thickness on the tube as well.
https://www.irwin.com/tools/drill-bi...eel-step-drill