I completely agree with wickedfrags.com. Seems to me the time to drill the tank is now. You'll have the tank empty at some point during the conversion, you won't have it empty once you stock it.
I used an overflow box for many years when first starting and yes they do work but yes they can fail in ways that a drilled tank cannot. Drilling is not hard and the holes can be easily plugged if you wish to convert to sump down the road (patch it over with glass, or just put a PVC cap in the bulkhead).
I guess to me it comes down to the KISS principle. To me drilling, although you have to first get into the right mindset of it I guess, is the simpler solution. Adding boxes and U-tubes or up-and-overs and etc. is the less simple approach.