Hey Mike,
I got my Montero with the original intention of chopping it.
I cut mine a little lower than most. I had intended to reuse the original top as well. By cutting at the lower body line, it left me more metal to work with under the windows on the top itself. I still have the top, but have pretty well scrapped the idea. There's a member here, T-bones, that hopefully will chime in. He's made his re-attachable.
As for finishing the top of the cut, I just made sure everything was nice and flat and straight. I then cut pieces of 1/8" flat bar out to fit, then welded it up. I did get a bit too much heat on the passenger side, but by the time I was finished with the body work, it's not too noticeable. I then sprayed the tub with spray-on bedliner. I applied it just like an "over-the-rail" job on a pickup. The thickness of the product really helps hide alot, and cleans it up nicely. It also gives me a nice finished interior, as I didn't want carpet in a convertible.
As for a top, I started by welding a channel to the windshield top just like a jeep uses. I got some vinyl from the fabric store and made a bikini top for it. It aint perfect, but it works. I am now getting ready to start making a hardtop for it too. My plan was to use 3/4" sq. aluminum tube for a real spartin frame. The cover it with 1/4" baltic birch 7 laminate plywood. A quick layer of fiberglass, then spray it with bedliner. This may seem tough, but between the wood and 'glass, compound curves can be built pretty easy, and then again, the liner will hide alot of imperfections, and leave me a nearly factory looking finish. When I get two pennies to rub together, I also hope to have a full soft top made. I really like the idea of the Rock Gear jeep tops. It can be a full soft top, or a bikini, without using a bunch of extra bars.

As for the cage! You will NOT find anything pre made! These rigs are enough of a different size that anything will be custom. I believe the rig that Lordtrunks has that used to belong to HighMtnDesert has a roll bar that started out as a hoop from one rig then had to be moddified.
Just find a quality builder to build you a quality cage. I'm about to start rebuilding mine as I'm not happy with either the look or the fact that I bonk my melon on it. It also needs to be moved slightly inboard to allow more clearance for a top. I don't wheel that hard, but I did build the truck for my family. I wanted something that was as good or better than the original top, which mine is. (with the exception of course, of the whole gettin' poked part) Mine is MORE than better than anything Jeep has ever put in their rigs from the factory. I know it will protect us in the event of an accident. I empasize that because this rig was not built for "hardcore" wheeling with the intention of putting it places that it is LIKELY to roll.
Here's a few pics



This one is before the tub and bedsides were sprayed. The rockers all already done.