I don't know anything about the JC Whitney dashes, but the ones like alot of guys around here run in their mud Jeeps like to crack really easily. Make sure you are supporting it right under the area you are working on, use masking tape, and run as fine of a tooth as you can on whatever you end up using to cut it. It's easier to go slow and take your time than it is to pay for another dash.
Just be careful with it. I've seen one go in the trash before it ever got to the Jeep. Depending on what's still in the way under the dash, I'd consider fabbing some sort of brace to go behind the passenger side. I'm sure it's not as thin as the ones I've seen, but it's still fiberglass.