put a level against the guards to the ground (and make sure its plumb). measure the distance from the level to the existing rim.
then go to https://www.wheel-size.com/size/ford/falcon/ to find the existing size/ offset of your rims.
then go to https://www.willtheyfit.com/ and enter the existing measurements, then the measurements that will make it flush with he guard if you want it flush. adjust for poke, or in a bit, or whatever.
I wouldnt go closer to the strut, and if your increasing rim diameter you might even need to go further away. pop your hand to the the top of the rear of the tyre and feel what kind of clearance you have.