Going over the top won't scratch the barrel. The barrel is narrower than the sight, that's why I was thinking along those lines. I was afraid tapping the shims in as you suggested would scratch the finish.I like the C-clamp idea. I was leaning towards something like that, but was hoping someone would chime in and say they did it that way and it works before trying it myself
I just tried doing the C-clamp thing with an old Blue Streak barrel that I don't care about (the bore is trashed). Put the wedges in place, placed the sight over them, padded top of sight and bottom of pump tube with hard rubber, then tried to squeeze the sight on over the wedges. It was getting pretty difficult to turn the screw on the c-clamp when I quit trying. Maybe it's do-able with a larger clamp. I removed the clamp and found that one side of the sight had settled into the groove on the wedge on one side and the other side was above the groove on the wedge but was pressed down hard enough to grip it. In other words, the sight was partially on, but tilting to one side and it would take more force than I gave it to get the other side to drop down into the wedge groove. I tried giving it a few raps on top of the sight (elevation screw was removed beforehand) but it didn't budge and I didn't want to hit it any harder. Like I said, this technique might work with a larger c-clamp than I had. Come to think of it, I didn't try to spread the sight open. If that's do-able (seems like it would take quite a bit of force- that's some pretty stiff metal) it might have made it easier to squeeze the sight in place.
I now make up a pair of plastic wedges and slide the sight on. I paint the soft white plastic black and it looks very much like original wedges. Various forms of plastic can be found at your local hobby shop.