The image above shows the JBL 1200GTi and the Shiva side-by-side.  The Shiva is definitely a deeper driver. It also doesn't look all that nice mounted facing inward. I opted to mount it facing outward instead.
 

Another picture comparing the two drivers.  While the Shiva might have a greater Xmax, the JBL 1200GTi has a greater effective diameter (and as a result, a greater Sd). This evens the field somewhat, as it's Sd*Xmax that determines the driver's peak SPL capability at lower frequencies.

The pictures above show how I installed the Shivas in the existing enclosure.  I had to fabricate new baffles, as the old ones did not leave enough clearance between the rear of the Shivas and the rear of the box for good air flow. The grilles are temporary, and were made to protect the drivers from anything else thrown in the trunk (e.g. groceries, my daughters' school bags, etc.). The redesign of the baffles raised the net volume to just about 1.1 cu.ft. per driver - this results in a Qb around 0.9.  I lowered this by adding polyester fiberfill to each side of the enclosure.

How did sound?  Again, quite well - it's difficult to go wrong with a good sealed alignment.

I plan to add a dress panel to the top of the enclosure to make everything look neat. Once I find the time :-)
 

Results
I was quite happy with the sound of this subwoofer.  It was quite a bit cleaner at lower frequencies than my previous sealed subwoofer design.  It also clocks in at 139.1 dB driven by 250W per driver - not bad for a simple sealed alignment.