You don't necessary need camber kits if you don't mind running increased negative cambers and wearing down your tires sooner. Your car will run fine without it long as you get an alignment after the springs are settled. The camber kits are to help your alignment guy dial your alignments back to oe specs and saving your tires longer.
You can stretch 275s over an 11 inch rim. Not recommended but it's doable.
All depending on your goal for the car. If you're going to drive hard, pick up a set of rear camber kits and wider tires.
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