Yeah, I was in the same spot you're in. My frame is rusted bad where the rear tailpan used to sit (it rotted completely away, the only thing left were the thicker sections that reenforce the tailgate support and the bolts to the frame (body mount).
I have ripped out my entire rear floor, cut out all of the rust in the front section (in front of the drop). I'm putting in 1/4" steel plate for a new floor (heavy, but solid!), I have a new tailpan, and I'm going to patch the front with the good metal from my old floor. I'm also going to extend the back bed to the back of the front seats, and use the floor area for the rear passengers as storage area (cut an opening and hinge it). The back seat is not going back in (not in the stock mounting place anyways), and I'm going to put in eyebolts (nuts welded to the bottom of the floor in various locations (8) and 4 eyebolts so that I can make different cargo carrying configurations, or remove them all if I want to put something flat on the floor (like a matress!!).
I had to replace my shocks, because they were rusted out. Well worth the money to do it when you have the floor ripped out, as it's easier access. I also dropped the gas tank, removed the filler neck, etc, sanded it all down (including the skid plate, but only cause I wanted it to look purty) and painted it all black, looks brand new again. I sanded down the frame where I could, still have a bit more to do, and painted it all with rust inhibiting paint.
I have to cut off the frame rails BEHIND the spring perches, and weld on new ones that I picked up (thanks for the tips found on here about welding frame rails), I'll have them sanded down and painted as well. I found a donor K5 that had been burned out, I grabbed the entire back end off of it. The tailgate posts on mine were bunk as well, so I am cutting off the entire bedside (inner and outer fenders) about 1/4 of the way up the wheel well, and grafting on the new ones (easier than trying to rebuild posts and mounting points for the new tailpan). Once that is done I'll remount the gas tank, put in the floor permenantly, and Rhino line the entire interior. I already did the inner wheel wells and they look great. My truck was in an accident (I figure) with the PO, and the entire front end was rebuilt, and a new crate motor (350) about 35,000 Km ago with Documentation to prove it (installed at local GM dealership). It'll be a sweet ride when done, and then I just have to fix the rest of the exterior rust (rockers, some on doors that just have to be sanded down, and front of rear quarters). Paint, and then start accessorizing.
I'm putting in all of this for less than $600 by doing it myself. A lot of hard work, but I love it, and it'll be well worth it to see the results when I am done.
I have seen cars with cancer sooo bad that the owner used OSB plywood to patch the damage. IMO, if your frame has large holes in it, and the floor is rotted out badly, its time to shoot it and buy another. depending on year, you can find a rust free one for relativley cheap if you are not looking for a show queen.
) for a non rusty one .......... and your buddy and yourself can lift the body off the frame without touching a single body mount bolt , then its dangerous 
If you can find a K5 in better condition ,buy it!It also depends on what you plan to use the blazer for.If you just use it as a winter beater then fix only what you NEED.If you are going to trail it then start cutting.If you want to restore it then buy a winning lottery ticket! 
