Thanks, Antony! Clearly, waiting for that tool would have been the right thing to do, but I get impatient at times. I figured the sooner I could fix this, the sooner I could stop stewing about it. We do have a machine shop at work, and it does come in handy! Of course, things always take longer than you think they're going to.
Bill, alignment concern duly noted! Fortunately, unlike my brain cells, the dies are self-aligning if I get them even reasonably close. The rounded tip of the male finds the countersink on the female and the dies freewheel into alignment easily. The spar is now completely dimpled, and no rivets were beheaded during the process.
Meanwhile here I've been re-reading some of the other previous screw-up chronicles and feeling a *lot* less alone. I met a guy a while ago whose homebuilt had a nicely engraved, prominently displayed placard reading "Don't do anything stupid."
Maybe by the time the wheels leave the runway, we'll have learned to pay a little more attention. Maybe the build process will make us better men.