The only one's you have asked about are called 1/4-20 x 1/2 long or 3/4 long. (quarter twenty). I assume they are machine screws not sheet metal screws. Some can be called bolts. The 20 number is the threads per inch.
20 is standard coarse. You may want to count the threads to verify. The head style is your choice. Should be available at any hardware store or big box home supply store. I have never ordered hardware. Maybe you are rural and it's a long way to the store? or there are other reasons.
As far as what sizes are needed in all the areas you describe, that is tough to say. If you make a list of what exactly you are attaching maybe someone with a 46 can identify. If you have some of each and can measure them, that's easy.
It may be time to get educated on hardware. Look up sizes, mostly coarse threads. Go buy one of each bolt/screw and one each nut. Use those to size threaded holes on the truck and/or match to the extra screws/bolts you have. Calipers and a thread gage would help. If something like a bumper is mounted with a bolt and nut, measure the clearance holes in the parts and get the hardware that fits with a little clearance. After a while you will know by looking what you need. The lengths are determined by the stack thickness of parts, including lock washers, flat washers and nuts... with a little extra stick-out past the nut. Some are used in threaded holes, that length is determined by the stack and the depth of the hole. This is a very basic skill that you need to learn ASAP. Asking for what sizes to use all over a truck is "arm chair" restoration/rescue/refurbish. This particular subject needs a "hands on" approach.
The exact truck does not matter. A bolt is a bolt. A screw is a screw.
You probably are only going to run into a few sizes 1/4, 5/16, 3/8, 7/16, 1/2. Mostly UNC, sometimes UNF. Small screws are numbered 6, 8, 10, 12. Grade 5 will be suffice.
If you get stuck and are not angry at me, post with any problem. I am truly helping you and want you to succeed. Anyway, it's the first answer. There will be others.
