I used M10 x 25mm allen cap head bolts, with M10 washer top and bottom.
I didnt remove the suspension, all i did was jack the car up enough for the front wheels to drop all the way down. You can then get in with a combination 17mm open end/floppy head socket spanner. When you mark out the holes for drilling, make sure the brace is centred on the top of the strut. If you dont, theres a ridge that the holes will straddle, and it will make tightening the bolts extra fiddly. All i did to get it to fit under the gas strut was to remove the gas strut using a flat blade screwdriver behind the clip, get a mate to hold the bonnet up, and bend the bracket away from the strut using a small adjustable spanner.
Hope this helps!!