I think this is an awesome idea! It will be a lot of work though.
The first step would be to pick up a clamshell and take everything apart (both comps) to see if things will fit. You'll have to build some sort of custom mounts for sure.
There's probably a whole heap of probs that you'll run into. I'm just going from memory of stuff I've read, but you will have probs with the trackpad and keyboard for starters. I don't think the trackpads use the same interface (the G4 being a USB) and the G3 something else ADB??I think). And I think the keyboards will also have different connectors. That means using the existing top half if the powerbook case, and modding it to have a similar look, and to fit the shape. I think that will be the hardest part.

ifixit.com has take apart guides so you can get a glimpse of what your getting into.
If you pick up a busted clamshell on ebay, and it looks like things won't fit, you can always resell it!

Good luck and take lots of pictures!