Re: Increasing rear ride height
The aluminum blocks, red spring insulators, and over the axle u-bolts are not stock. Looking at that set-up in the photo, you should be able to get at least 1.5" lift by going back to stock. If you do plan on multi-leafs in the future, you will get some additional lift. I can't answer the question regarding shock extenders, but you won't need them if you start by removing the added equipment.
I am not sure if the wheel tire combo you are looking at will fit. The tire seems awfully big to try to fit without rubbing. With 4 7/8" backspace you will likely see rubbing on the inner fender well with articulation of the differential. I'm sure you have done research to make sure they fit.