Hi Ian,
It is not an easy process...once you gut the interior, you'll see the metal frame. It needs to be in good shape. The floors are 2 inches of marine grade plywood. You'll want to check that for rot in such a wet climate. The shell mounts to the frame in the same spots as the bed would...I added some extra bolts as well and big steel plates for washers. It all needs to be spaced properly, left/right and vertically, I used hockey pucks as vertical spacers, 30 of them in total, 15 per side, 3 per bolt. Then you'll decide on where to cut out the back of the cab. The shell would move around a lot around the cab without some modification. We welded extra support struts from the internal metal frame, near the front, to some plates that bolt to the top of the cab to better tie everything together. Its very stiff now, but theres still a bit of movement at the seal between the cab and shell, its visible as cracks in the paint, however it is completely water tight, unlike the old chinooks.
Finally, when I put this thing together I bought two old chinooks (you can find them for free sometimes) and used all the best parts off of both. A lot of the moldings and little parts are just plain hard to find. The windows can be a problem too if they're in bad shape.
Good luck!