Mike,
I mounted dial thermometers (don't remember the brand) in my Bullets in the same location you're describing. Mine are about 3" from the bottom of the dome on the opposite side of the vents. They're mounted high enough so their dials don't touch the ground when the domes are put on the ground.
Truthfully, if I were to do it again, I'd mount them higher on the dome, probably somewhere at the halfway point or above.
Don't go with a longer stem. Even with the 2 1/2" stems in mine, there are times when the probe will touch the meat if I'm not careful about where I position the dome in relation to the meat.
Be aware the reading on your new thermo will not be the same as in the center of the top grate. But, by comparing that reading against that of a trusted digital probe in the center during a test run, you can simply adjust the target on the dial thermo to compensate for that difference. In my case, for example, to hit 225 in the center, my target on the dial thermo is 240.
Another thing I did when I installed them was to rotate their faces so that when they reading at my desired target temperature, their needles would be straight up in the 12 o'clock position.
With this approach, I can tell from a distance how far they are from the target temperature. If the needle is toward the 10 o'clock side, the Bullet is lower than may target. If it's to the right of 12, it's running hotter. I'll call this the High Noon Method for measuring cooking temps.
Hope this information helps. Enjoy your WSM.
Ken