hi i just done the same with my 58. the bolts are underneath since i was stripping the dash for painting i took the dash apart, and it was still a bear to get to the bolts. good luck.
I did this a couple of years ago. I swapped the old mono speaker with a stereo speaker. It was actually 2 little speakers attached to a frame the same size/shape as the original mono.
Yes, you have to do it from underneath. I didn't take the dash apart, but I recall that I took the radio out to give myself some more room for navigating up there.
It can be done, but you might scrape a few knuckles.
Scott where did you get the stereo speaker from? Any modifications for wiring have to be done? My father in law is looking to change the speaker in his 66 Beaumont.