Lifting the springs is not what controls tire clearance. The bump stops control compression travel. With spring lift, it "seems" there is more clearance, because with a static articulation test, things sit higher. But, one day, in a dynamic situation, they will compress until the bump stops hit. That is where you must check clearance.
Now, some spring lifts will actually fully collapse the spring before hitting the bump stops.... This is a bad design as it is hard on the axles and hard on the springs. People really should remove the springs and check articulation clearance using only the shocks and bump stops. At the same time, you can check that there is room for the springs to fit without fully collapsing.
The factory spacers are around 1/2".
I'm pretty sure a 305/70/16 will hit and require some minor flare trimming and longer bump stops in the rear. A lot depends on the wheel used.