Honestly, not that bad. You already have a plasma cutter and MIG. Get a small cutoff tool (like 3" cutoff or a regular 4" grinder with cutoff wheels and a dremel for the nitty gritty stuff.
Cut out areas that are more or less rectangular while the truck is on jack stands (take as much load off of that area before you start cutting). Remove inner fenders and other stuff that may be in the way. Use cardboard (beer boxes work great!) and a sharpie and scissors to make templates for each piece you cut out. Doing it in bigger pieces is better, but I've patchworked several frames and unibodies (XJ and MJ) with good success.
Make sure that where you're welding to is solid metal. If it's thin/rusty/iffy....it's gonna suck to weld to. Also, clean the area as best you can with a wire wheel before welding....clean steel is easy to weld to, dirty steel is not.
Lastly, youtube is your friend. Look for "rusty frame repair" videos. You can weld in heavier metal to the thin unibody. Lots of people have done this with good success. Good Luck!