I'm not sure about an exact dimension but the closer you locate it to the axel the less problems you'll get. I know there are a few here who can give you a better idea on the distance between the bottom & the ground.
Thanks for the replies, I was wondering because I kinda designed my build after TQ's style, and his firebox's look really close to the ground in the pics. I guess I will go between 8 and 10 inches....but hate being indecisive about stuff...LOL. Thanks again
Made mine 1" above top of axle.
But did weld a scrap of 1/4" steel to the bottom of the air intake/clean out,that happens to be the lowest point of FB.
Here's a pic or two.