After much thought about all of the suggestions, and after much thought about my initial stupidity, and after many nightmares about turning my lens into "parts" if anything went wrong, I decided to "fix" the lens.
The first photo shows the bottom of the built-in tripod mount. The small holes to the left and right ot the mounting hole are original. They are shallow and were unthreaded.
Tp drill and tap the hole on the left had too much potential for turning the lens into "parts", so I left it alone.
The hole on the right was in the "overhang" (see the second photo), and the careful drilling of a pilot hole, then tapping had potential, and with a great deal of caution, I could drill all the way through. I did. Finding a proper length 10-32 screw wa no problem because I NEVER throw anything away in my shop. The #21 drill bit, not so easy, but a 5/32" bit worked. After drilling all the way through the "overhang" on the mount, I tapped it and the mounted the gimbal plate. (See the thirf photo.)
Problem solved?
In so far as mounting the plate to the lens and eliminating the "spinning" the lens wants to do when mounted at one point only, YES!!!
Top heavy still, even with the body attached, and the plate mounted as far back as possible. I will look for a longer plate.
Now all I have to do is find a very strong young person who wants to hike to where the birds are lugging this 13+ pound monster for me.