I don't know if there are specific distances written down anywhere, but I'll give you approximate measurements from my '48 half ton.
Are all three supports the same? No. The rear one is definitely different. The forward two appear to be the same.
How close does the running board get to the front fender? On mine it's about 3/8" at the narrowest, but this could differ from side to side or truck to truck, based on how the front fenders are attached.
How close is the filler panel that bolts to the top of the running board supposed to be to the back of the cab ? Again, this can differ, based on how the bed is positioned on the frame. Mine are really close - maybe a quarter inch, but I've seen other that were up to a half inch.
How much space is there supposed to be between the bed and the back of the cab? At the center of the cab, I've got a half inch.
I hope this helps.
