Are stainless steel bolts ok for holding the the clutch cover? I need some M8, 15mm bolts and I can find stanles steel. But not the hi tensile bolts I was planning on using. What grade of bolt should I be using for this job?

High grade bolts only for that function, if you have ever seen a clutch cover go through a car you wouldn't think of using anything less than OEM or ARP.

I driver got extremely luck years ago, clutch cover came through the transmission tunnel over his legs and jammed in the door half way through.

It is actually not a particularly demanding application , the shear forces transmited are not that high. Grade 10, 10.8 or even grade 8 , 8.8 bolts are fine.
I would not however use stainless I would use use Allen screws 12.9 which are easy to find in this size on the internet or eBay.

Use a suitable grade of Loctite on the threaded.

Yep, to back up BT here it's a fairly tame loading but worth getting decent quality parts anyway, not just making do.

As above, grade 8 or higher. Not worth the worry fitting anything less/cheaper

M8 16mm grade 12 bolts have arrived so I should get every thing fitted this weak end.