Rick, this is something I've been trying to figure out too (Ya, ya, no runner... But it WILL be... MUAHAHAHAHAH <IMG SRC="smilies/biggrin.gif" border="0">)...
I have two suggestions. First off, SOMEONE makes a Toy pickup removable-hardtop kit. Apparently I can't find it on the net, but it's there. You could at least use the front of this. The rear would have to be customed with some fibreglass (Unless you know ABS well; I don't. <IMG SRC="smilies/biggrin.gif" border="0">)... Should work fine.
What I'm planning on doing is cutting it at the line of the bed (which will be the line of the body w/o the top) but at an angle (30 or so degrees), so the metal slopes down towards the outside of the truck (towards you if you're standing there), and welding on some plate on both the cab and the roof. I'm gonna use a 1/4" disc (or something <IMG SRC="smilies/biggrin.gif" border="0">) so I can cap it with 1/8" sheet, and then I'm gonna use some thin foam or rubber on the bottom of the roof piece...
The front will be a pita, that's why the kit would be nice. But, I think if you can have the front of the roof piece overlap the windshield frame, it won't leak. Still figgerin. <IMG SRC="smilies/biggrin.gif" border="0">
My 2 cents.
...I also want to try to make the windshield removable. But I'm nuts. <IMG SRC="smilies/biggrin.gif" border="0">