I've got a 98 Dodge Ram extended cab, and am looking to have the box under the seat. I don't really plan on removing the seat, as I often have other people with me. I'd be willing to raise the seat a couple of inches in order to put a ported box down there, and as for which woofers, I have no clue. That was my next question after I found a box.
As always I would say take some measurements and build yourself a box. By lifting the seat a few inches you should be able to get a half way decent enclosure in there for a pair of 10's or maybe even a pair of 12's. As always with trucks though I think mounting depth will be your primarily limiter and not airspace or mounting area as in other situations.
I've never really been around car audio in my life, so what is the audible difference between sealed and ported? I don't really care about how loud it'll get, I'm just concerned with the lowend extension.