As soon as you change your engine you screwed for insurance. There are companies that do it however they charge for it.
Check with the transport department for your state on the requirements for certification. From what Norbie said, it sounds pretty straight forward in Qld to do small modifications. Bare in mind that with an 18R-GU you still need to sort out EFI fuel and wiring, along with new exhaust and throttle cable. I would still look into the 1G option. Physically it isn't that much more work. Getting it certified might be a little harder though.