I'm assuming that your existing cutout is for a radio with turn knobs on the side, like this:
But you want something like this:
The simple way to do it is just cut a whole with a sawzall. If you've got a nice stock CJ and don't want to cut the dash, you may want to go a different route. Maybe something like this:
wow that looks nice! Guess I'll get the sawzall out and start cutting away!! No this tub is about shot and not worth saving!! Just looking for a summer ride and do a winter restoration!!!
You will prob need a install kit so it looks nice like the yellow jeep pictured, or make a strap of some type to hold the radio in so it don't just fall out of the hole you cut. I have done both, the install kit was nicer though, there's like a cage that holds the radio in the dash.