#1. I wouldn't imagine that a 2" spring originally intended to surround a coil over would be a suitable substitute for a factory spring. The torsional stresses that it would get exposed to going aggressively around corners would have me believe that that kit was intended as a drag race modification.
#2. I wouldn't do anything to a car, including a major surgery like a minitub unless the little voice in my head, and NOT the rest of the voices on this forum told me to do it. While I cant speak for them, I believe everyone involved with this thread feel the same way. If you don't want, or need the additional tire capacity then don't do it. If you do, it can always be done later. (You just gotta pull all of that junk back out of the car, and undo some of your shiny, and sticky mods).
#3. Whether you buy a QA-1, or a Strange shock, you can get them for around 160.0 ea. at Summit. Springs add another 50 ea. so you're right, a little over 400 buys the pair. On the red car, I used the stock shock mount, and I only had to add spacers to shim the shock, and stand it off the rear end so that the spring wouldn't make contact with the axle tube during axle wrap. The top mount you have pics of. I welded a plate w/ a nut to the inside of the relocated stock shock mount, added a 3/4" spacer and ran one bolt through the top shock mount to secure that. After thinking about it, I added that goofy stud onto the spacer that went through the stock shock mount on top to give the mount additional support. It leaned in the 1.250 we're talkin about here,..just enough so it didn't make contact with the frame rail. I drove that car about 6000 miles the year that I had it after I finished it. The way that was setup worked perfectly.