I don't have a 250, but rather a F150 crew. I did 2 kicker CompVR 10's down facing in a sealed enclosure under the back seat. I mounted the amps to the back of the cab behind the back rest of the back seats. Like you, I wanted to keep as much functionality of the back seats as possible. The two things I lost were a couple inches of foot room in the back and the ability to "lift" the back seat up. To remove the box/subs, I have to take the back seat apart. I also have to take apart the back seats if I need to get to my amps for any reason. Neither of those have been a concern since I did the system 2 years ago. The foot room would be the biggest concern, but I don't think I've ever even had more than 2 in my truck at a time regardless, so it's not an issue for me.
Two things I did on this install that I've never done on any previous vehicle or boat, was use kicker subs/amps and dynamat the whole cab of the truck. I went with kicker stuff just to see what it was about as I've done just about every other brand out there. I noticed from this board that more and more are using it and praising it so I thought I'd give it a try. I must say, for the price, I'm pleasantly surprised. I don't know if their stuff is really that good or it's the fact that this is the first vehicle that I've dynamat'ed. Having to pull the entire interior out of the truck was a pain and made for the most labor intensive install I've done, but it is WAY worth it. I did all of the surfaces since it was a truck cab, floor, doors, roof, rear, etc...It really makes the system pound and a pleasant side effect is that it eliminates all outside noise going down the road as well. My truck didn't have much road noise to begin with, but now it's like a damn hyperbaric chamber in there. With the system just absolutely pounding, you can stand next to the truck and barely hear/feel anything. That is ultimately what I was going for by trying the dynamat because as I get older, the more I hate hearing other peoples music. Now at stop lights, I know people aren't cursing me.