I don't know why you would ever need 2 12 l7's in a pickup
I just did 1 12 l7 in a slot ported box 3 cu/ft @ 32Hz and it is very loud and fairly acurate.
Depending on what kind of music you listen to I would suggest the following:
Hiphop/techno/rap (synth beats) = 1 12 l7 dual 2 ohm (load 1ohm to amp) as big as you can get the box behind your seat with tuning to dealer spec (per above @ 32Hz)
Rock.. (actual guitar) = 2 10 l7 dual 4 ohm (load 1 ohm to amp) subs share volume of box, as big as you can get the box behind your seat with tuning to dealer spec.
AND
something comperable to an Orion 1200d (1ohm stable amp)