The number stamped into the firewall behind the master cylinder is also required and checked for by vicroads.
The numbers stamped into the firewall behind the Masters is a production/build number used by the factory and is not relevant to registration or Vicroads. The VIN/Body number stamped on the radiator shroud is the one that matters and should match the compliance plate if one is attached. A compliance plate is required when first registering the vehicle and if not available can be replaced by previous registration history, but it is a good sign if it matches the number on the radiator.
In terms of legalities the largest single piece of the vehicle is what used to get assigned the VIN/Body number so in a Mokes case it is the shell and there was at the time no matching of other parts of the vehicle to that VIN so you can put whatever you like on there from other Mokes and except for the engine and changes to the colour you don't have to notify Vicroads. The desk jockeys at Vicroads really don't know what they are looking at as I have seen examples VIN stamped into a plate riveted over the radiator shroud that have passed.
A statutory write off vehicle is considered to be damaged too severe to safely repair normally involving an accident and insurance companies but it doesn't really cover rust bucket Mokes that probably shouldn't be repaired unless it is involved in an accident and the assessor deems it is not able to be repaired, otherwise you just have to get it past a roadworthy inspection.
In terms of your '81 with 1977 front sub-frame then the only real concern there is how do the bumper bars mount as the '81 comes under the body while the '77 comes through the front panel, otherwise there is not difference.
Terry