you should be able to drill the back plate as long as you know its not tempered as well. i would drill further up the over flow and if you have the room outside, make an external durso pipe like on this page:
http://www.rl180reef.com/pages/stand...ug_closeup.htm
If you don't have the room outside but you have the room inside the overflow, put the durso inside
check out the info on this website for more info
http://www.rl180reef.com/frames.htm
Is your overflow already built in?