OK,
Let me ask a couple of questions.
What kind of contour/mystique do you own? and does it have fold down rear seats?
I can tell you my plan with my car (1966 Chevy biscayne) to give you some ideas, but my car has some advantages based on the design.
The first thing I will be doing is cutting/installing the baffle board. Since I will be using Dayton IB12s (3 or 4 of them), and they have less than 3/4 of an inch of excursion, I will probably be using 3/4" material and mounting the woofers behind the baffle. I would fit into place and drill the holes necessary to bolt up to the bracing behind the rear seat. Then remove it, add some bracing (just like i would to a sealed/ported enclosure) and pre-drill the mounting holes for the subs. Depending on how permanent i want this baffle to be mounted, I would then either glass the perimeter to the body of the car, or just use some expanding foam around the perimeter and put a layer of dynamat around the perimeter and possibly covering the baffle. Then add the subs and finish material.
Then I would be adding some bracing to the rear deck and dropping a layer or two of sound deadening sheets. The bracing is to stiffen up the flat pieces of sheetmetal and the sound deadener will add mass to decrease the resonant freq.
The next step in my case is to add a couple layers of deadener to the rest of the truck and apply a finish.
