Lopping the roof off - legalities

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Not sure exactly where this kinda question belongs

I have an old '65 Galaxie 4door sedan


It was purchased originally for a RHD conversion for a hardtop
Its been stripped of its trim, badges, and glass (all sold off)
the door trims, seats and steering wheel are missing
I do still have all the front panels and grill/lights






And I was curious about the legalities of hacking off the roof as far as rego goes...
I'm assuming this kinda thing requires engineering certification before rego ???
but being a full chassis car which doesn't differ much from the factory convertibles I don't think it will effect rigidity at all
It would become a dedicated convertible with no retractable roof (and stored in the garage during inclement weather)

Apparently the factory '65/'66 convertibles just had an extra crossmember in
the separate chassis near the back... which shouldn't be to hard to fab up...

But do I need to have windows in the doors?, or can i weld up the tops of the doors too and lop off the window frames
the rear parcel shelf would be filled in too

A small block v8 and auto on LPG would suit cruising during the warmer months

I was just thinking of making basic door trims and maybe some fibreglass mouldings for kick panel/pillar areas

Are there any other things I should consider ?