The Firestone site says that an 18.4-38 has a loaded radius of 31". You could get the bottom of your hanging weights around 28" below the axle if you realy wanted to get the most advantage you could.
Some say that the weights being able to move forward 1" doesn't make a difference worth bothering with.
The weights below the axle move forward 1". The weights above the axle move backward 1". That's 2 inches difference, which might mean 1" further pull. Sometimes in life an extra inch makes all the difference :)