Men...mission accomplished! I ended up drilling a 5/16" hole in the divider trim piece. As suggested, I did not need to hog-out the rubber gasket. I threaded the mirror stem into the threaded hole and used the jam-nut to secure it in place. There is no noticeable wobble; will test her out when April arrives here in Michigan!
http://s160.photobucket.com/user/st...nder_zps637f53b1.jpg.html?sort=3&o=0Thanks for all the good advice--as usual. There is always something new to learn about these old, lovable trucks of ours!
Happy New Year to all!
