Why not just duct the hot radiator air into the low pressure area on top of the hood, like the
GT40 or
Motorsport Elise do? It looks like you have space in your engine bay, and it should help aero, especially if you button up the underside of the engine bay.
As for your question about sealing the front of the car up and moving the radiator(s) to the rear, it should help aero, but you are going to have to duct air to the new radiator location(s). Where are you going to get the air? Tire well? Under the car? Side scoops? Roof scoop(s)? I think the execution of this ducting would play a big role in how much improvement you see, but that's just me thinking.