oh dear , well you may have some room left for packet of crisps or drink in the Z
id say sell the big amp and l7 is your best bet , get a smaller mono amp matched to the sub wont cost that much.
i looked up those shallow rockford subs, nice kit wish i had seen them before i went for my 8 as all the other shallow mounts out there dont take the power , infact that sub would match my alpine amp spot on.
either way good luck it should give a good install .