I've owned,patched and driven many K5's and GM trucks that were much worse off than yours--up here your truck would be described as "minor rust for its age"...all but a few of the many trucks I've had ,were worse off than your photos,and needed patching just to pass inspection..(main reason most of them I got were cheap,people just parked them after they were rejected at inspection and didn't feel it was worth fixing)..
My first few trucks I owned I had only an arc welder ,a pop river gun and I also used self drilling sheet metal screws,and nuts & bolts, to put galvanized steel over the rot holes,metal I used was the thicker stuff used on large freezers I found in at the dump's metal pile,was 16 gauge..
My floors were not pretty--but solid as stone,once everything was spot welded and screwed down,and I painted over everything with POR-15 or Rustoleum,then rubberized undercoating..
Eventually I bought a small set of torches so I could cut and braze metal and get rusted bolts cut off--after I got a few trucks under my belt I got pretty good at using the arc welder at low amps to spot weld patches in,or I'd use the torch to braze them instead...
I have yet to own a MIG,never had the money to justify one ,though one with the ability to use argon/co2 would be a major improvement over my crude methods..but it can be done other ways,like I did my first few trucks..
I was not out for show quality,or going to use the trucks for off roading,just street use,so what I did was sufficient to get them to pass inspection,be safer and more comfortable to drive,and keep water from gushing in when I'd go thru deep puddles..
All the trucks felt much more solid after the floors and rockers were done over,the doors no longer rattled,and the body was not loose on its cab mount bolts and cushions any more..my 72 K5 was the most noticeable "before & after",it felt like a solid,quiet new truck compared to how it was previously..
When I put the floors in most of the trucks I used one large piece to go all the way from the firewall to as far back as needed to replace any missing or rotted oem metal..the less seams,the better,less places for rust to begin again..
I used thick enough metal so as not to get that annoying "oil can" popping or flexing when you drove over bumps..some trucks had thin aluminum roof flashing used as rot hole patches--it made it rust more due to dissimilar metals making a battery effect,and added no strength at all,I had to rip all that out..galvanized I used wouldn't rust again in my lifetime,but welding or brazing it can get you ill if your not careful,so I did it outdoors and avoided the smoke it releases..
I also wasn't afraid to add more braces where I thought some should be or were factory,and flat stock 1/4" thick under each seat mounting bolt to beef that area up to prevent any cracks..
For cab mounts on my '72 K5 I used the bases off old bumper jacks I had instead of buying the thin cheesy OEM type sheet metal cab mounts ,I cut those right out,welded the jack base to the underside of the floor,in place of the original,the jack base was nice and beefy,had a hole the cab mount bolt went thru and the rubber cushion sat perfect where the jack's upright post would have gone..
I also used things like electrical outlet box covers as huge washers to go around the cab mount bolt onto the floor,those are fairly thick,galvanized,and already have a knock out for the center hole..they also worked well under each side of the cab mount rubber cushions.
Where I used nuts & bolts,each got a washer on both sides under the nut and bolt head..
I had a ton of 1/4" nylock nuts so I used them under the truck...
Like I said,my end results may not have won any prizes as far as looks,but the floors and door bottoms I installed using these methods held up a long time,I kept several of my trucks over 10 years and daily drove them,and when I sold them,they were still solid where I had patched them..
The worst part about rust repair besides getting cut up on sharp metal,is the time you'll need to do it--I did my trucks over several summer weekends,and crawling under them to screw on nuts and try to tighten bolts alone sucked ,I had to use vise grips and was lucky I have long arms to reach many of them--once it was all over with though,the satisfaction lever was well worth all the effort..and it did not cost much either,most of the metal I got for free,I spent more on welding rods and nuts & bolts and paint than anything else really..
