A potted history of the early cars is in order.
In 1947 Toyota made the SA as essentially a VW Beetle with a water cooled engine up front - the public hated it.
Toyota put the same engine in the ladder framed small truck called the SB.
The public loved this and made custom sedan bodies for it.
For the next 10 years Toyota released variations on this same chassis with various sedan bodies on it (SD to SF, RH, RR).
This chassis was sometime customised (by the factory and others) to receive the B and F engines and special bodies (fire engine, police car, ambulance).
I believe (but can't prove) that the BJ also used a variation of this chassis and engine.
There is a good reason why Chinese call white ambulances 'death cars'.
For starters, the colour white is the colour of death.
Imagine being in an accident and then waking up in a small white room which feels like it is being shaken by demons, there is a horrible wailing sound, you're tied down and some people dressed in the colour of death keep trying to stick needles in you.
- Stepho