At this point, I wouldn't worry about the weld. That is unless it's also welded on the bottom flange of the frame. That's a big no-no, unless some sort of reinforcement is added. Welds on either the top or bottom flange of the frame will weaken it, and the spring mounting points are some of the highest stress areas.
The parts book doesn't specify what the taper of the shims (original PN 474682) should be, but those don't appear to be stock. They could have been installed to compensate for sagged springs sometime in the truck's past. With new springs, you should probably go with some original shims.
