The Falcons were out there but remember during the fifties and sixties GMH commanded almost 50% market share and Falcon is a child of the early sixties. Before then it was either an expensive Ford Custom line or tank Fairlane/Galaxie 500 all of which were low volume luxury
(read expensive and somewhat harder to find even if you did have money) cars. The Ford alternative to Holden was the Zephyr, or you might have a Morris Oxford, but the Holdens were everywhere and usually the first things to go were the badges anyhow.
Like Henry's model T Ford, which is the car that got hot rodding rolling in the US, Holdens were both plentiful and cheap, so they too became the raw material of choice.
This is purely speculation, but considering it wasn't really util after WW2, that is, during the 1950s, that all the US Hot Rod and Custom Car publications began to proliferate Australia.
So I think it's fair to suggest Aussies were not really exposed to the customised pre war cars as were our US brothers and it was the fifties stuff that got us rolling.
But boy, once we figured out what was possible, those cheap and available Holdens were being souped up and/or chopped up, all over town.
The drag racers used them for exactly the reason the hot rodders hated them, they had an integral chassis and were incredibly light, so it didn't take much to make them competitive/winning race cars, especially on the drag strip.
For example, at the last summer night meet in Sydney early in 1968, John Walker's Ford Sports coupe
(I may be wrong here but I think it ran a Y block) took a class win with a time of 16.59 at 80.78mph whilst Tom Stranger grey powered Humpy ran a 14.91 at 88.49mph to take win his class.