Since that overflow box has slots near the bottom that also allow water in, the level in the tank with siphon down to the level of the standpipes when power is off. If you had a normal internal overflow with just slots/teeth on top the tank would only siphon down to the bottom of the teeth on top of the box or to your return nozzle, whichever is lower.
If you have sufficient room in your sump to safely contain several more gallons of water you could shorten the standpipes the extra inch or so you need, otherwise I wouldn't attempt it.
It is easy to calculate how much water is contained in an inch of your display, LxWxH(in this case 1" or whatever you determine you need)/231. In my 60"x18" 100 gallon tank 3/4" equals 3.5 gallons and 3/4" is how far it drops when power goes off. My sump is a 30G that normally runs about 2/3 full so there is plenty of room to safely contain the extra 3.5 gallons. You will need to do a little math or experimenting until you feel comfortable.
BTW, I have used a single Stockman standpipe for 14 years on my current system and on others before that and never once had a single incident. I even push it to the max with a Water lasted HY-5000 pump running wide open.
