So I'm guessing you moved your springs in? My measurement from drum face to inner side of spring is 6". That would limit backspace to something under 6".
I get 5 1/4" from drum face to inner fender for a total spring to fender width of 11 1/4". I'm gessing your tires are about that wide.
Inner side of spring to frame is 3 3/4" extra space with spring relocation.
Those Darts look great set down low. I'm figuring what I'll need to achieve a similar stance with my Demon clone.
Something I wonder about is how all that rubber will limit the ability to light up the tires. I'm planning a 360 magnum built to about 450 hp.