Going to be doing pretty much this same thing with some Image Dynamics XS-65's but have been trying to come with mounting idea's...wondered if you've got anything behind that woofer for support...baffling etc???
My current "work in progress"...
Clarion NX-501 H/U
Scoshe double din adapter
Alpine PDX-5
Alpine MRX-F30
Alpine R series sub w/enclosure
Alpine SPR-68's (rear fill)
Image Dynamics XS-65 comps.
The woofer is mounted to 1/2" PVC mounting board which is in turn mounted heavily to the factory plastic. Everything around/behind the mounting area is Dynamatted and it works pretty darn well.