I have experience with my `66 Belvedere II. Best bet is to remove the glove box and the gauge cluster. A bit of work in themselves, but the overall experience will be much better. With these two items out, the dash pad swap is a cinch.
You may be able to unscrew the glove box and just lower it behind the dash. It took me some work (breaking it down) to actually get it out and back in the dash. There was not enough clearance between the dash and heater box to drop it out the bottom. You just need enough room to reach up under the top dash lip to access the pads' mounting nuts. If you do need to take it out, you will need to get all the screws out and manipulate it out the front. I laso removed the door to get mine out.
As for the gauge cluster, screws from the front hold it in place. You also need to remove the heat/ defrost knobs and cover plate. Knobs have small allen heads on the underside of the knobs. Remove the speedo cable and any electrical plugs you can from underneath. Some of the upper plugs you can get from the top when you roll the cluster out.
The gauge cluster has a bit of a pinch point at the steering collumn (at least mine did) as you roll it out. To lower the collumn, remove the plate underneath it, gaining access to the studs on either side that hold the attaching collar. Remove the collar. The chrome trim that goes over the collumn needs to be removed as this is what's causes the cluster binding.
Once you have got this far you should be able to disconnect the rest of the electricals from the cluster and remover the cluster. Now you can easily get to the remaining dash pad nuts.
While you have it out, this is a great time to separate the bezel from the gauges and clean them up. a soft paint brush and compressed air will make them look like new. If the needles are faded, some florescent Orange paint can be applied with a model or small craft brush. When it all goes back together it looks like new!
Take photos before you disconnect something so you will have references incase you don't get it all back together right away. I am giving you this from memory, so if I am out of sequence, or forget something...well, I've slept (once or twice) since doing mine. Good luck, and don't rush yourself.
