It would work just fine, but keep in mind that the bed of a pickup is separate from the cab. So..... take the pickup bed off (troublesome but worth it to do it right). Build your enclosures to the cab of the truck, seal well, and use METAL, Dynamat inside very much. Make sure they are sturdy, but the weight of the sub should be mounted to the cab first, then the enclosure to the back of the cab. This enclosure doesnt have to look great.
Next, measure how much you will have to cut in the front of the bed, then cut it out. put the bed on, then build yourself a new front to the bed to cover the hole and the sub enclosures. Make sure there is clearance between your cover and your enclosures, as the bed is meant to torque on the frame when loaded.
If you do it right, it will look nothing more than a tool box, and you will get sound that is unmatchable from behind seat enclossures.