Sierra Ext Cab

 Can't remember how much room is under the seats in one of those, but the ususal method is to install 10" subs in a custom box underneath them.  I've seen some guys who have hard tops for their beds weld the bed to the cab so it won't twist, make a cutout between the bed and cab and install subs firing into the back of the seats (expenive and requires a lot of time and experience but sounds awesome because you can accomodate up to 18" subs this way).  One guy I saw who did this had 3 Kicker 15" solos and my ears rang for about an hour after hearing it from inside the truck.  If  the front of the seats comes down all the way to the floor then take the seats out and see how much space there is.  Some trucks have massive ammounts of space under the rear seats and nobody realizes it while other lack enough room for anything but a couple of 8" subs.  If there is a lot of room then I would reccomend four 10" or three 12" subs that can each handle 300 or more watts RMS (Alpine type R, mtx 8000 series, coustic carbon fiber series, ect) in sealed boxes and amps that match their power rating (hey you said Mad Bass right?).  Pricey but really loud.

     The seat lifts are a manual modification as far as I know and good steel and high strenght bolts are a must.  A machine shop will be able to suggest what type of steel and could make you some really cool lifts for a pretty penny.

     I'm not really sure what it would cost to do the bed weld and cutout.  Most of the guys who do it have the equipment and know-how and do it themselves (I would be tenative about doing it myself).  You should find a respectable body or steel welding shop in town and ask them what they would charge.  Remember that the passage between the bed and cab must be water tight and remove only as much material as is needed to (removing to much weakens the frame).

      Another note is that you won't be able to abuse you pickup as much once this is done because you might bend your frame or break the welds off.  The bed was intentionaly not attached to allow for greater body flex under rough conditions.

     If you have an idea of what equipment you want to install I could give you a couple diagrams of what the cutouts should look like and possible box design types.  Saw one really cool one that had a plexi-glass on the top of the box so when the bed cap was open you could see the backs of the subs.

On the seat lift thing I don't know how 1998chevy1500 did it, but below is how I would suggest it be done.  a space should be recessed for the rubber washers that isn't quite as deep as the washers (the washers are to seal it watertight).  You could create a lift of any height with this method and basic shop tools(a drill press, carbide tipped metal drill bits, and a die grinder).

THIS IS A SETUP!!! tho i dunno if you want to shell out the cash for the W7 and 500.1 JL amp. lol