Welcome to the 1969-72 Blazer/Jimmy world!
Get some more pix of the truck so we can see what you are working with. Many on this forum have been down the 'rust repair' process of these trucks over the years and can tell you that the rust you can see - is only the start.
There are a few steps to do before you get too far into cutting/fixing things.
Is the factory top on and bolted down? If not, do that first.
How are the body mounts and bolts? If shot/rusty, do that second.
How do the doors open/close at the bedside? If they stick, fix that next.
How are the door hinges? Loose, door dropping, fix that next.
Now...you are ready to start the rust repair stuff as, if you completed all of the prior steps, now have your truck very close to the factory specs to rust repair.
I'm guessing from the pix you posted there are rust concerns for the following areas once you start digging in (structural items are marked with a *):
- floors on both sides*
- a-pillar on both sides*
- kick panel on both sides*
- floor mounts on both sides*
- rear floor supports on both sides (B-pillar - under floor)*
- outer floor on both sides*
- inner rocker on both sides*
- outer rocker on both sides
- B-pillar on both sides
- B-pillar support to bedsides on both sides
- lower bedside on both sides
- *maybe on rear torsion box end caps on both sides
- *maybe on torsion boxes on both sides
I used Key Parts to fix almost all of these areas over the past year and I purchased almost all of the parts from Jeg's (which offers free shipping on most parts except doors/hoods) - so you save a ton of money there if you don't have a place local to you to get replacement sheet metal.
As noted, post up some more pix as we can provide more information as needed. Here is a before/after on the one I just fixed up (still working on panel gaps).