While I haven't had a crack in the dash, I did have a crack in the panel that covers some of the electrical. It's the one on the right hand side, copilot side in front of the steps. What I used to fix it was JB Weld. It's a two part epoxy that is gray when dry. I sanded both sides of the crack and then applied the JB Weld with a plastic spoon and some popcycle sticks. This worked fine since I could lay the panel on it's face and work with the back laying flat.
It would be a lot harder to do with the dash in place. One suggestion might be to make an aluminum or thin wood strip about 2" wide by length of crack to cover the crack, then put the glue to stripe and place over the crack. Most likely you would need to have some heavy medal weights over the top of the crack and some wedges from below to hold the strip in place until set. Just an idea!
Good Luck with this one.
Great Salt Lake Route
2004 Alpine 40 FDTS