Stock tires are 265/70/17's (31.8") approx. 10.5" wide.
Stock rims are 17" by 7.5" wide with approx. with 5.75" backspacing??
The first oversize is 285/70/17's (33") approx. 11.5" wide, which most Z's can handle without touching a thing. You might want to crank up the stock keys. New rims can be used, but if they are wider or have less backspacing, they could pose minor rubbing issues. These issues can be generally solved by trimming or torsion bar crank. Any rim over 9 inches wide is not recommended due to rubbing issues.
The second oversize is 295/70/17's (33.5") approx. 11.5" wide, which will require Ford Keys, perhaps rear spacers and a little trimming. As with 285's, different rims will play a role if how much trimming will be needed or perhaps suspension tweaks(more crank, adding rear spacers, etc.).
The third oversize is 305/70/17's (33") approx. 12.5" wide (equivalent to a 33x12.50r17). These will require spacers, Ford Keys adjusted to level, and I would plan on a good amount a trimming. As stated above, different rims will play a role if how much trimming will be needed or perhaps suspension tweaks (more crank, adding rear spacers, etc.). Because 305's are so wide, the gap between the rear wheels and body and other components is extremely close with stock wheels. Under heavy loading or suspension articulation, the rear tires may rub. Flipping the rear-upper shock mount bolts is a good idea, this will eliminate a lot of rubbing. New wheels with a different backspacing or width will help limit the amount of rubbing in the back drastically. Any wheels over 9" wide or less than 4.5" of backspacing will cause rubbing issues in the front, anything else should be fine. A popular tire is the Goodyear MTR, this tire in the 305 is known to be bigger than the typical 305, 34" tall generally. This tire may need more trimming or lift.
The next size up is a 315/70/17 (34.5"-35") approx. 12.5" wide (equivalent to a 35x12.50r17). To clear these tires, the vehicle will require a minimal of a 3" body lift, trimming, and a torsion bar crank would not hurt either(Ford keys would be even better but are not necessary). A 6" suspension lift by itself will clear these tires no problem, minor trimming up front may be required. As stated above, different rims will play a role if how much trimming will be needed or perhaps suspension tweaks(more crank, adding rear spacers, etc.). Because 315's are so wide, the gap between the rear wheels and body and other components is extremely close with stock wheels. Under heavy loading or suspension articulation, the rear tires may rub. Flipping the rear-upper shock mount bolts is a good idea, this will eliminate a lot of rubbing. New wheels with a different backspacing or width will help limit the amount of rubbing in the back drastically. Any wheels over 9" wide or less than 4.5" of backspacing will cause rubbing issues in the front, anything else should be fine. With a body lift, 315's on a 17x9 seem to be a good setup with almost no rubbing with some trimming. Even with a suspenion lift, 10" wide rims have been found to cause rubbing issues up front.
. 285 WHAT!?! or what are your rpms at 70mph with 285s? alksjdflkjasdf?! 
wtf
