If your not going to drill the tank then you need what is called an overflow siphon box. You'll need at least one.
http://www.jlaquatics.com/phpstore/s...ct_ID=of-cs050
(Not pushing this brand. Included merely for illustrative purposes).
Basically the water is siphoned to the sump and then you have a pump in the sump that pumps the water back to the main tank. Size of overflow and pump depends on size of your system.
Cheers,