The 55 gallon standard tank is typically 100% tempered and can't be drilled. You could replace a pane of glass but disassembling a tempered tank is risky and you could fracture one or more panes of glass, probably easiest to just smash the pane you plan on replacing.
Before you go ahead and purchase a hang on overflow consider getting a new tank, plenty of good deals around on used pre-drilled 50 to 75 gallon tanks around. A 55 long isn't a great size for saltwater anyway.
For example:
http://www.canreef.com/vbulletin/sho...threadid=69326
Probably cheaper than a decent HOB overflow