Usually, you use flexible hose to bring water from the sump, via
powerhead, to the main tank. At the tank entry point, you could build an inverted U shaped rigid pvc and attach the flexible hose to one of its ends. Make sure you drill a little tiny hole above water line on the side that brings water back into your tank so if power is off the returning siphon that now will take water out of the tank, back into the sump, will be broken when air is sucked in.
Post some pics of your Wet/Dry. 300GPH is quite good for handling 75gal tank with a Wet/Dry sump filter.
Pepe
Santo Domingo
Edit: never mind, I saw your pics on the 29gal sump in the other thread.