Im in the midst of my restoration as well..have done the rear frame rails, shock towers, immer and outer wheelhouses, inner and outer tail panel, drop offs and extensions as well as both full quarters..welding in bracing and taking measurements are extremely important..there are conflicting rear frame rail measurements floating around, so getting them while the originals are still in is important. As far as the shock towers go, there are two sections, the structural mount and the pan section itself. My mounting sections were still good (still have the replacement sections I bought that I dont need) and my truck was in decent shape, so I just cut out the pan sections and but welded them in place. They come with no holes in them so placement is easier, and then you just drill out the holes from underneath using the structural tower holes as a template..makes proper placement of them a cinch.
As for frame rail replacement, like I said measure, measure, measure, the must be placed right or your rear will be all out of whack. As for welding them, unless you are en experienced welder with the proper tools I would ask a pro to do the actual welding or be sure to practice a lot before attempting it...they are a critical amd structural part of the entire rear of the vehicle. Need any help/pointers feel free to email me.