What make/model truck is it going into, any ideas on your maximum dimensions?
Would you consider making or buying your own custom enclosure instead of the prefab ones you linked?
Do you have an amp already? or will you be purchasing an amp based on your sub selection?
I would look into something like this
http://www.onlinecarstereo.com/CarAudio/ProductDetail.aspx?ProductID=17537 or
http://www.woofersetc.com/index.cfm?fuseaction=product.display&Product_ID=4993
Mounting depth will most likely be your biggest limiting factor, if your willing to get creative you might be able to get a beefy sub in there, probably looking at somewhere around 2-3 cubes of space and around 6" max for mounting depth.
Of those two that you linked I would go for the kicker setup.
well, to be honest, I thought that a prefab box would be the way to go seeing as how I know very little about choosing a box for a seperate subwoofer. It is going into a 1996 Chevy full size regular cab truck. I have a Planet Audio 4 channel 600 watt amp. I don't know what all the cubic feet dimensions and sizes mean, and how to match a sub to a box.