None of the wheel studs or rear axle to brake drum studs are splined. They should be smooth sided with a D shaped head that locks against the hub or brake drum. Like
THIS or
This. Front wheel studs and rear axle to drum are 42mm long, rear wheel studs are 37mm long.
If you've used a splined stud and it's turning, all you can do is grind off the nut and press out the remains of the stud.