18'' wheel how big or a tire can i go

[usarmyoif3]

ok i have a 71 stepside im going with 18'' torg thrust 8'' fronts and 10'' rears im going with a 235 or 245 tire on the front on the back i want to go with the widest tire i can with out narrowing the rear end or tubbing i have a 5 1/2'' front drop and 5'' rear drop with a c notch does anyone know how wide a tire i can go with out running into clearance promblems and what kind of back spacing should i use? thanks alot guys once again.