You may need to separate the wiring so only the amber lights handle the flashing and the red stay as tail and brake only. Sometimes changing the flasher relay or controller can fix it. A bit of rewiring should make it compliant for WA.

On a 1969 Mach 1 check the VIN on the driver side dash (visible through the windscreen), the driver door plate and the inner guard under the bonnet. Some cars also have it stamped on the engine block or transmission. If you still can’t find it, VicRoads can guide a physical inspection.

With high 400hp a properly built C4 with upgraded clutches/bands and a mild street shift kit is a solid choice and helps it live longer. I would avoid the super harsh/full manual style for a street car, but a good builder can keep it feeling nice while still strong. You can also keep the stock shifter for the classic look and just upgrade the internals.

Heat the emblem with a hairdryer, lift it off with fishing line and remove any leftover glue with alcohol or adhesive remover.