I was going to suggest checking to see if you left a socket in the clutch spline.. Seen people do that before.

Change to bolts, M8x1.25, you'll have to check the length though. If you're stuck i'll measure mine on the 8v and tell you what you need. 8.8s (tensile) would be fine, you won't need anything harder. Don't worry about stainless IMO, just get plated 8.8s.