OK you are my god
Thats EXACTLY what I was planning to do!

Any more pics you have of that would be more than welcome...

Im planning to run one of these on either side of the boot space and box in the space behind the rear speakers too.
Maybe just moving your attached box a little further into the boot space so that the side plastics in the boot are flush. If I can find second hand plastics, they'll probably be removed altogether and the boot padded out more, as the ABS box gets in the way.

Im looking at Pioneer TS-E1777's in the rear (fill), Infinity components 505cs up front, and infinity RA6006 amp 4x 50W RMS and 2x75W RMS to drive the 2 Alpine SWS-1040 10inch subs.
The jury is out on the subs as I am torn between these and dual voice cone drivers for 2ohm ability, or twin 8ohms and bridging the amp.

APEXi comes later

Were you running your current amp through that attached sub setup?
Any ideas on the transfer function of the rex?