I've got two questions. First does anyone foresee any problems with running
255/45/20 F and 315/35/20 R? Second question is what is the proper torque for the lug nuts?

Thanks

Clay

100 ft lbs. is the torque number....

You wouldn't want 275/40/20 in the front? That's the biggest closest match to the rear size you listed.

Front and rear tire diameter must be within 3% of one another. Need to know what brand tires, as different brand tires of the same size can have different diameters.

Front and rear you want to be as close as you can get. Front 29.03" for 255/45/20
and 26.68" for 315/35/20. That is almost a 1/2" diff, that will eat your t-case up over time.