You will need a super high efficient woofer or find space for an outboard amp. Although the channels may bridge, a head unit just doesn't have enough room for the size power supply to generate ample power no matter what is stated on paper.

I would also suggest maybe a single large driver if you can make the space work, like a 10". A 6" just doesn't ring "sub" to me. Could provide some fun midbass punch though.

Does the head unit have a built in crossover? If not, there's another thing to consider.

A sealed box would work best in small spaces but sealed boxes require more power. Also, less math to do with a sealed box, just find a woofer designed for sealed box and match the minimum cubic feet. With a ported box you'd have to figure your cubic feet, pick a woofer and find a program or formula to tell you what size and length of port you would need.

It's been a long time since I've done any car audio. If Infinity still makes the Perfect sub series they were very good in sealed enclosures. One of those with a small Rockford bridged and you'd be slamming. If budget is there I actually like Precision Power amps better. Then again, due to space and heat, for your project if considering an outboard you should really use a digital amp.

Why not put a box in the bed with four 15's and run the port into the cab