I've done radios in our four XJs, and get the best results using the original (Clarion?) radio's DIN mounting bracket. After removing the dash bezel, remove the two screws-one on either side of the radio's faceplate-and slide it straight out. The bracket can then be removed from the original radio, and with little or no modification be installed on your new aftermarket head unit. I like to use the original bracket because of the hook on the top, as it fits into a slot above the radio, offering better support.
I also like to use an adapter for the wiring harness, as it not only makes wiring easier, but also allows options such as dimming with headlights, or a power antenna, to operate properly.