Johny,

Wow, that's steep huh...anyway if you can get a tiller on that ground then yes tear it up at least 9" deep...the deeper the better. I like purple stripe and porcelain. One suggestion for you is to leave a grass strip between the areas you want to grow in, this will cut down on erosion. I think you can plant it on the slope just fine..the bulb won't care.

being that it's this late you can cover the ground to be planted with some black plastic to gain some heat for maybe a week before planting then be sure to cover the bed with a healthy layer of mulch to retain that heat.

I hope my advice helps you .

Bloosquall Garlic Gardens
Waitsburg Wa