You must have a minimum 70mm X 70mm X 2mm steel washer under the floor that the seat mount bolt goes through, same for seatbelt mounts. Check with your local engineering certifier but I'm pretty sure lap seatbelts are still allowed. You then need it to be certified by your engineering place and have a modification plate fitted, then present it to your local rego office to have the details updated.
Pretty easy really.
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