Be aware that the anodized layer is an insulator, and you'll have to cut through it somehow to get a chassis ground. If it's not too thick, a star washer may do the job - otherwise, a bit of dremel work may be in order.
On most Bottlehead kits there are two chassis ground points - one for safety near the IEC socket - this one is important!!! The other is the signal ground, usually the center terminal of a T-strip near the first stage.