Sorry for the short reply earlier, I was queuing for some food at a show. I used aluminium ones on all of my car as they are easier to set than the stainless ones.
If you look carefully along the top edge of the bulkhead you can see how many fixings I put in, they went straight into rivnuts on the scuttle lip. These rivnuts were put straight into the grp and I haven't had any problems yet (only done 100miles).
I also ended up using m6 stainless hex head bolts as its much easier to get a spanner into most places than an allen key.
Both rear arches in position. by
Series1Landy, on Flickr