Rim & Tire Size Help:

[ridingreiss16]

Please help!
I was just told you cant lower 15x8 rally rims in the front because of the spindle clearance and front tires rubbing, so I have to go to 15x7's in the front. Thinking about just selling the rims and going to Weld Racing. If I do 15x7 Weld Racing in the front, what is the biggest back rim I can do in the back and the fattest tire I can go without tubing the bed? Truck is almost finished painted and and I need the rims/tires ASAP. Thanks!