Ya, you always want to dry fit everything together first. How do you plan to run the pluming that a cock valve will be able to control the water level in the pipes. Weather the
pump is running or not, the cock valve can only adjust the amount of water flowing through it (not control water height unless you open and close it manually every cycle). If it's in the fill side, water will still siphon back to the reservoir through it until the water level is low enough that it begins to suck in air. If it's in the overflow side, it will only restrict the water flowing back to the reservoir. Thus cause it to back up in the pipe while the pump is on, and possibility overfill. And when the pump is off it will still siphon back into the reservoir through the pump until it's low enough that it begins to suck in air. Water level in a
hydroponic system is usually adjusted by the height of the overflow and flood line openings (cheep and easy), and the height of them is easily adjusted as your plants grow.