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I would still precast using GFRC. At 3/4" thick your looking at 360 lbs plus sink. Flipping it will be your biggest challenge. Make sure to use a good mix should be fine. Bigger GFRC pieces have been made. It will also depend on the access you have to get it into its final place.
I would suggest to you to make a smaller piece first and then tackle your big piece. If you have never done one before its a good idea to practice first. And watch as many videos as you can.