JL 12W0-8 ohm load issues

[Pasyngr]

My neighbor who is a diesel mechanic for heavey equip. wants to put in a amp and sub that he has into his rig. he has a Lanzer vibe 418 amp. and 2 JL 12W0-8 subs. the prob. is that there is only enough room for one sub. I know that with 2 I could wire them in parallel to get the load to 4 but since there can only be 1, is there a way to configure it to get the 4 ohms or is it safe to run the sub normally with this amp. also in regards to a box for it I was thinking because of space ( there is only about a 2x2 area between the driver and pass. seat) I could build him a down fire box for it since it is a work truck , this would protect the sub better, but he likes the SPL so if I understand it right that means a ported box would be better. Is is a ported down fire an option? I know that these are pretty rookie questions but I've read all the faq's related to subs and ohms but still dont quite got a grip on it yet. any sugestions?