You don't have to disconnect the speedo from underneath, but you do have to disconnect from behind the cluster. There's a bunch of wires and such that are connected to the cluster (about five that go to the bulbs behind the "idiot" lights along the bottom). Just take it slowly, and try to remember which goes where. If you have some masking tape handy, you can make little tags as you go. To remove the cluster, you want to have the correct tools-or make a pair from coat hangers. Here's what they look like (they're the T-handle thingies):

http://www.harborfreight.com/cpi/ctaf/displayitem.taf?Itemnumber=97901

They slip in between the dash top and the sides of the cluster, about an inch or so from the bottom. Slide them in and turn 90 degrees to engage the sides of the cluster, then pull out (gently).

I always keep in mind that these cars are built very well, and are easy to disassemble-- incredibly good design. The important thing is to never force things, and to try and figure out the little tricks-- where the hidden screws are, etc.

Good luck-

Dave