What I used for bushings is simple rubber vibration isolator pad material (the pad was approx 12" square), up here we have a *Princess Auto* hardware/automotive supply store, Harbor Freight *might* have something similar, using my table saw I cut the bushings to the same basic shape as the original molded bushing glued to the leaf & removed the material in the inner section to make the *pocket*. One has to do them in two pieces (upper & lower).
One can *fine tune* the installed height of the new spring by adjusting the upper bushings thickness, but both bushings have to be no thicker than necessary to allow the spring to go into place & tighten up the lower mounting retainer to let it bottom against the stop.
Seeing as the blank leaves are 3" wide (that's wider than the glas leaf in the centre section at the mounting points) I used my 5" angle grinder with a flap wheel to notch small recesses into the edges of the steel where the pads will go to allow the mounting bolts to slide up either side of the leaves up into the subframe member. All of this wasn't difficult, it just took time & patience.
If you've never removed the rear leaf without the special spring compressor there is a way to get it out without to much fuss.