Scrape off the loose stuff. Open the factory drain holes (they probably have a very hard, deep filled in area). and get them open so water can't sit there. If you can get it clean enough for the rust converters or paint to stick, they can help some, but paintable products does not replace missing metal, it only aids in keeping the metal still there a bit longer. I suspect your going to find a few extra holes in several of those areas as you clean off the crusty loose stuff.
As bad as its going to sound, the fine line test for the condition of the metal is the pick hammer test. The old ark welding slag removal pointed end of the pick hammer is one of the best metal thickness tools available. The test is pretty simple. Using the pointed end, swing the hammer into the metal you want to test. With driving a finish nail hard swing, at the suspected weakest point of the metal, you will know for sure what shape the metal is in within seconds! If the pick hammer bounces off, or puts a small dent in the metal, its good. If the pick end blows through, the metal is bad. If the hammer blows through, the metal is oxidized enough you can't weld it. Period! You can use the pick hammer to determine how far you need to replace the metal to. If you don't believe me, take a 3" square of 20 gauge sheet metal and hold it over an opening, and strike the center of the opening with the puck end of the hammer. You can hit 20 gauge sheet metal really hard with that pick hammer and all it will do is bend the 20 gauge metal. If the hammer blows through, that metal is thinner then 20 gauge. If its on a frame rail, its not much support. If the pick hammer dents a frame rail, I would deeply consider patching the frame. I would caution the use of the pick hammer test if you really don't want to patch the holes, because I promise that knowledge of the condition of the metal will never leave you.
I fixed this kind of stuff for 30 years at my welding shop here in the north west corner of IL. I'm retired now, so no I won't fix it for you, but I will advise you on how you could fix it.
Of the 3 pictures you posted, none are critical structural areas except maybe the flat area on the right side of picture #1. That is the frame support above the lower control arm. If removal of the crusty metal reveilles small holes (1/4" or smaller) in that support frame section, I'm still not sure I would be too concerned, but if that area had larger holes, I would consider adding a metal patch large enough to add support to that frame ear. Do the pick hammer test. After you clean it up, and get to real metal, if you are concerned, post new pictures and I will advise from there. You can PM me, or email me.
I would expect the pocket with the rubber bump stop to have the front edge rusted through. Its just a bump stop, and it doesn't appear it has been used in a very long time. I would not be concerned at all. The frame under my 49 had rust holes in one of those pockets. I cleaned them to bare shiny steel (with the rust holes intact) and painted both sides. Those pockets now have painted rust holes, the good news is, they don't hold water anymore.
The pocket are in the last picture should have a drain hole in it, your probably going to have to really work at it to get it open. The fine dirt and sand will be like breaking a chunk of cement. getting the drain hole open would be good, but probably not life threatening if you can't.
The area's of larger concern are the boxed frame behind the front wheels. Look up inside the factory holes in the bottom surface and pull out all the chunks of frame you can from inside those boxed frames. That flakey junk inside is holding water inside the frame rails. The factory holes are suppose to be for draining water out. Once cleaned, if its really thin or has a lot of holes, the bottom section of the frame can be replace with 1/8" plate steel. If you replace that bottom, be sure you remove the old holey metal, and expect to be welding about an inch up the side walls. Also, be sure to put some drain holes in the new piece. If its still intact, that frame patch may not be required for the useable life of the truck.
The next area of concern would be behind the trans crossmember where the frame converts from the boxed metal to the "C" channel. That "C" channel has a 1/2" to 3/4" lip standing up from the bottom inside edge. That extra lip allows water to stay there until it starts rusting the lip, and the bottom of the channel. The bottom of that channel can also be replaced with 1/8" flat metal, but don't add the inside water holding lip back on. You may have to weld up the outside about an inch from the bottom there as well. Gene