Even is the sump is only slight lower, it will work.
I have used both barrels and tanks, and as long as there is a slight bit of gravity going , once you start the sumps water will be pulled from the main tank.
Here are a couple examples that will work with either overflows or drilled.

on this one you'll see the overflow box is sitting on the edge of the sump, only 6-8" below the tank.

next a barrel on the floor, where the rim is only a couple feet below the tank.

and it doesn't;t matter if the sump is off to the side.
Below is a sump off to the side, ao access to equipment is easy for maintenance, and water entry to the sump, is only inches lower than the tank surface.