You can put the filter anywhere that you want. With power filters, I generally put it near the end of the tank and put an airstone on the other corner back wall. That way it gives it some circulation. You could also put it in the center of the tank. Just make sure you can install the heater with enough room.
I prefer mounting heaters horizontal about 2 inches from the substrate because it is easier to hide and IMO
keeps the tank heated more evenly.