overflows are fine you just have to be carefull to establish min and max levels in your sump, the min would be the level just before your return pump starts sucking in air, the max is a little more complicated to figure out but not by much, once you get the water flowing through the system at just a little above the min level in the sump put a mark on the glass and shut off the pump and let the water siphon down from the tank, if you have the overflow set properly the water should only come down until the siphon stops(you should also put some sort of siphon break on the return line to keep water from back siphoning into the sump) once the water stops filling into the sump measure the amount that the water rose from the min mark placed on the glass, for example we'll say the water rose 3", now you know if the power or pump fails just how much the water will rise, now add a comfortable amount of leway to that say 2" and measure down from the top of the sump and that will give you the max level for the sump, as long as you keep the water level in between the min and maxhave a siphon break on the return and the overflow is adjusted say @ 1/4 below where you want you water suface to be everything should be fine, now I know I've rambled on to long, so I hope this helps you