I know it has been done several times; if you are going to just ditch the top altogether, that makes it even easier. Even if not, cutting the top is quite easy. Just very, very messy (think drywall dust).

Aside from teh top, teh part that makes the Runner different is that the "bedsides" are hollow (well, normal truck is too) but the inner side is not a complete wall. E.g. when you take the plastic panels off the interior, there really is no reall "wall" to that inside side. So depending where your cut line is, you'll have to make up some framework on the inside. My suspicion is that you'e end up making new "panels" from wood/MDF or something anyway though.