Easiest way to get at it is from the radio opening. If you've got a blanking plate in there, pop it out and reach down from there. If you've got a radio, you'll have to take it out. That's still easier than trying to get at it from underneath.
(And I know, because the choke knob was loose in my '66 -- I had to tighten the nut that holds it to the back of the dash -- and I had to take my radio out to get at it.)
Yep, it's a bit of a bear to get at, that's for sure...