The first thing I did was measure the thickness of the pack wall at each mounting point then place the bracket on the motherboard accordingly, allowing a 1/4" play or so just to be safe, then mark the outline on the motherboard. Then, one by one, taping down the brackets and test fitting, and re-test fitting the shell on the board until the placement of the bracket seems close enough. It should be said that the the bracket doesn't have to sit right up against the inside wall of the pack so don't stress if it's slightly off.
After you get all the brackets placed, clamp them down and drill 2 holes into each bracket, going all the way through the bracket and motherboard. Then either use the appropriate size screws and bolt the bracket to the board or for GB1 accuracy, use 3/16" large flange(head) rivets like I did.
once all the brackets are securely attached to the motherboard, place the shell on the board and make the proper adjustments until its centered to your satisfaction. Now, get a #7 drill bit and drill right through the side of the pack and into the L-bracket, making sure to go all the way through the bracket leaving a small clean hole.
Now, with the pack shell still on, insert a 1/4-20 tap through the hole in the side of the shell and carefully cut threads into the L-bracket, keeping the tap as straight as you can. You wont be able to see the actual bracket while you're tapping it but this is okay because when you tap with the shell on, the insertion angle created by the hole in the pack will create threads in the bracket at an ideal angle to insert the bolt you will be using to secure the pack to the bracket.
Here's what you are doing:
after tapping, insert a 1/4-20 socket head cap screw or button head cap screw(Socket head cap screw for GB1 accuracy, Button head cap screw for GB2) and tighten to a loose thumb tight. Then drill and tap the rest of the brackets in the same manner. Be sure to consult the great reference section here for bracket/screw placement before drilling!