You don't want rear axle oil leaking onto your brake shoes and drum.
That won't be a problem. The axle flange is outboard of the brake drum by quite a bit. Any leakage will be on to the wheel, not inside the brake drum.
As far as thickness, that's not critical. The only thing that would change is the depth of insertion of the axle splines into the side gears. It would take a really thick gasket to make that too shallow to be a problem.
