Thanks 4RExplorer. Actually, I decided to go home that afternoon of my post to try on my own and it was actually no problem at all. To anyone hesitant about the project, don't be! The point of concern - the brackets attached to the door frames - detached from the seats with a bolt reachable after popping the seat back plastic off (which is only held in place by easy to use clips that don't break; pull from the bottom). Then the side bolt holding the bracket on is easy to reach. I would recommend taking some pictures of the 60% seat's seatbelt buckle arrangement, as that would probably be the only tricky part putting it all back together in the future. I also recommend putting the seat bolts back in the holes to prevent liquids or dirt from getting in and below that panel. I should mention that it was also self-explanatory how to remove the transition piece with the tie-downs in back of the seats. I decided to cut the insulating fabric underneath the gap in the styrofoam insulation, so I could remove the front piece of styrofoam that would get in the way of any attempt to build a level plywood cover. Cutting the fabric in this place should not have any negative repercussions when reinstalling the seats later.
Now comes the harder part of cutting a perfectly fitting piece of plywood to make the top of the new storage box. To get the floor even from the back of the truck to the back of the front seats will result in the plywood cover being about 8.5 inches above the floor in back of the front seats, which should leave a nice bit of storage underneath.