That's gonna vary depending on what tires you're running, tire
pressure, fuel load, conditions of springs, etc. My 3604 is
an example. Although the springs and the frame are the
same, as a 3100, the axle and tires are not. I would expect
it to be higher than a 3100. The outer bumper bolts bolt
C/L is about 17 1/4 on both sides.
Check out the three pictures at the bottom:
http://www.pbase.com/dennygraham/1950_general_body_shots&page=allThe outer bracket would be about 7" from the front of the fender.
If your using the reproduction brackets they are easily bent
up/down/in/out by clamping a long bar/lever to then. I think
the original brackets may have a little more spring to them.
dg