Here is the trick to releasing the cluster bucket from the steel dash frame:
The bucket is designed to be inside (towards the firewall) on the bottom lip and outside (towards the steering wheel) on the top.
Pull the top of the bucket inwards (towards you) - as far as you feel comfortable with. Then get your fingers down under the bucket and push up on the bottom until the lower edge can clear the lip of the steel frame. Then pull the bottom of the bucket inwards.
It may take a little finesse, but at some point you'll have the top and bottom popped out of the frame.
BTW - it's way easier to unclip the cable from the back of the speedometer after you have the bucket clear of the dash frame. Clean and lubricate the cable sheathing at the firewall penetration and push some of the slack through the grommet. Then, when you start to pull the instrument panel into the cab, the cable will come right along with it. When you have room for your hand - unclip it.
Recent discussion on this same subject:
http://www.gmsquarebody.com/forum/showthread.php?t=9238&highlight=speedometer
