I have areas all through my truck frame and beyond that are hit with rust. My mechanic friend says the rig is road-worthy, just a whole lot of rust.
It has over 200,000 miles / spent whole life in the Northeast. I recently acquired the truck and want to try to get many more miles. I want it to be as strong as it can be and want to know:

1. What are the main areas of attention I should give to ensure frame doesn’t give out or other disaster?
2. Many bolts are very rusted. Is it a good idea to try to replace as many as I can to make it easier to work on worn parts?
3. Rear bumper is rusted through and want to know what is a strong and inexpensive replacement?
4. Corrosion on this part and don’t know what it is..(2nd picture)

 

So what is going to rust out the fastest? From my experience the spare tire carrier had the most rot followed by the fuel tank strap slots. Also your leaf spring hangers will probably completely rust away if they haven't already. Other than that the frame was seasoned with rust everywhere but not anywhere near critical failure.
You can strip the rust off the frame using a needle scaler and painting it with POR-15, but this will be very time consuming and will require some disassembly to do it right but with 200k miles it probably isn't worth it. The other option would be to spray the underside with woolwax or something similar to slow the further progression of the rust. This will be the most economical. As for the bolts, I wouldn't bother touching any of them unless you actually need to remove them, and if they are rusted enough to where you cant reuse them, replace them with stainless hardware and be sure to use anti-seize on everything.