Go Bobby! It always helps when a pro steps up!
And thanks for telling us what you are actually working with. The deep scratches can be a problem and I can understand why you''ve had conflicting helpful comments. The truth, as I know it, is that it will depend on how deep they are. If they hit metal, and I'll throw this in with the other rust you're going to deal with; There is a product labeled 'ONE-STEP' that comes in a spray can. It converts rust to a black looking primer sealer for areas that arn't logical for cutting out for replacing. As far as I know, other than POR products, there isn't much out there to trust.
So for know, you can detail clean your entire hot rod with a power washer/start removing anything that would hide crud/ bag it & tag it and maybe even take pics as you go so you'll remember what hardware goes where/PREP-SOL down the surface before you lay down sandpaper or you'll sand in old wax, armor all, grease, and other contaminants into the paint which will leave what's called 'fish-eyes' in your final paint.
If you don't have access to a cheap paint gun that you could experiment with on an old hood or a couple fenders, you could even watch a painting video that shows you proper gun techniques, and then use some old cans of spray paint to practice moving side to side without rotating your wrist, keeping the exact same distance away from the surface (tape a one foot ruler to the side of the can). It sounds corny, but an hour of prepractice before you shoot your first primer will allow your mind to concentrate on what you're seeing instead of what you're needing to do.
Are you going to color the door jams, fender edges, and such?
And as was pointed out, there's very little reason to ever go much heavier than 400 grit on a color recoat as you want to do. When you pick you're products you may find that the product dosn't even want you to step up to a final 600 grit.
for you small scratches, rust areas, welds, and such, they make professional level primers in spray cans that can be had at an automotive paint supply store: Don't use hardware store primers in a can. Those are for wheelbarrows.
The longboard sander is your friend. Watching a video on the proper way to use one is half the job. Don't. Don't. Don't. Do NOT D-A sand your El Camino. It will look like you painted a wheelbarrow. DA's are for body work and panel forming/shaping until you have a lot of experience with them under your beltline. They will ruin straight lines, pull out crispness, and leave little divots that don't show up till the clear is on.
Did I mention on my first post that this can be a whole lot of work??!!!! But they paint gun much easier than many vehicle surfaces- You'll do fine. Just watch a quality CD/Vid, do your prep correctly & cleanly, and if you know the right way to mask off with tape, you're ready to hit a home run.
Don't be put off when you get 2 or 3 suggestions on how to fix an area, or which grades of sandpaper to use when. It can be done a couple ways and different professionals have their own styles that work for them. It's like getting in a girls shirt. You do what works for the both of you and leaves you both happy afterwards.