Want a Static Level Drop - Need Help!

[stepn67]

I've got a '67 stepper which will have 255/60-15 meats all the way around and I want a drop which is level all around with about a 3" gap between the top of the tires to the fender lip. Does anyone have an idea of how much drop front and back I'll need to accomplish this? I plan to use a dropped spindle in the front with shorter than stock spring but odn't know how much. I looked at the examples of drops in the ECE site but all have a rake. Any help would be great. Thanks!