[BigBlocksRule]

Pull the gauge/control surround piece, that'll give you better access to that area. It might help to pull the gauge cluster for better access. Get some sort of small clamp - something with decent width jaws that will allow you to clamp directly across the crack, I've used Stanley Quick Clamps. JB Chemweld dries to almost the same color as your dash. Mix it up and make sure you get some in the crack, may have to use a toothpick. Smash it together as much as you can with your fingers, then wipe off the excess that runs out of the crack on top. Clamp it - use wax paper to keep it from sticking to the clamp - and let it set overnight. When you pull the clamp off, mix up a little more and apply it across the crack on the underside to give it a little more strength.
Done, costs less than $10 and you won't have to mess with the nightmare of removing and replacing that dash. Believe me, that's something you don't want to mess with if you can avoid it.
I know a guy who busted a connecting rod in his numbers matching 454 in his Chevelle. It knocked a clean chunk of metal out about the size of a 50-cent piece. He had everything cleaned, then JB Chemwelded the piece back in place. It's been at least 10 years since he did this and it's still running 11's.