they are cast headers as you discribed.
Land Transport NZ allow new bodies and vin an rego plates may be swaped as long as you are swapping like for like i.e. 4 door for 4 door. you must be the lawful owner of both vehicles.
i am luckly that i have the orginal ownership papers that say' Datsun 1200 SSS'and the numbers match.
as for the paint. no SSS was exactly like the other. some sills were painted (unlike mine), some window surrounds were blackened (like mine) the bonnet was always pianted. there were 3 SSS badges. air cleaners will have been changed in30 years . no idea what the orginals look like , mine has Ramco.
the car was passed in if a reserve of about $7k.