Bob,
We tried the brass punch with both a rubber mallet and an actual hammer. The mallet was't budging the thing, and the actual hammer moved it clear across the sight channel.
Since you say it's an SOB to assemble, I may have to go to plan B. Allow me to explain and see what you think.
I plan to take a 2x4 or something, and drilling a small hole in it. Then, I plan to screw a large set screw or something in it (this will all take place on the upper left corner of the board). Once I've done this, I will back the set screw out. It's only purpose was to cut threads in the wood. Next, I'll have to go to Home Depot and see if I can find a nylon screw of the same size to put in that hole.
Then, I'll just place the rear sight over that nylon screw and hammer away until the sight contacts the screw, which acts as a stop to prevent over-drifting of the sight. Fire the gun to see if it's any better. If not, turn the screw in a turn or so, hammer away with the punch some more, and try again.
You follow? Sound like it would work? Nothing else has so far.
Thanks!