I've recently finished putting a new body on my jeep. I went with the aluminum body....rather than fiberglass...because I never want to have to replace it again....but here's just a little of what's involved. There's a lot more to it than you think. Since no jeep is exactly the same...not only will you need to cut new holes in the firewall, you will need to cut the hole for the transmission & transfer case shifters, cut the hole for the gas filler and heat vent, drill the body mount holes, holes for the seats, rollbar, windshield frame, hood hinges, mirrors, doors, and several other miscellaneous things that I'm sure I'm forgetting. You will also have to replace the wiring from the firewall back to the dash, fuel tank, tail lights, etc....and with a fiberglass body, it will have to be properly grounded. Another thing you might think about on a fiberglass body....is how you will bolt your seatbelts in. I'd probably mount a metal plate up under the floor....and bolt through it. You don't want the belts ripping out of the floor if you get into an accident. Also, this job is going to take quite a bit of time....it's not just a simple matter of yanking the old body off and slapping the new one on.....when I did mine, I thought I could have it done in a couple of weeks.....and ended up spending at least 6 months on it. Things always seem to go wrong.....the torx head bolts used to hold on every body part...inevitably snap off....or a brakeline will get crimped.....or something. You'll see what I mean when you start yours. I hope this helps you out. Anyways....good luck with your jeep. Take your time and do it right....you'll be happier in the long run.
Don't play leapfrog with a unicorn!!!