You can do all that if you want but it'll probably never be approved for rego, unless your engineer doesn't find out about it. If you have a SWB buggy with registration history then keep the floorpan as original as possible to make re-registering it easier. If you're trying to register a new (no previous rego history) SWB buggy, then it would have to pass ICV inspections and be engineered at great expense.