Should've added the CPU in the loop. Those radiators should be sufficient with the CPU block. For easy drainage, use a T block with a drain port. Optimal placement should be at the lowest point of your loop, gravity will do the rest.
You might want to lay the RX240 on its side while filling and bleeding. There will be air bubbles stuck in the bottom pan of the radiator. Placing it on its side will allow the bubbles to rise out from the radiator.
The CPU at default clocks is 95W. The graphics cards are 210W each. 2 x7970's are 420 W of power dissipated into a loop, that is a lot of heat to remove. The CPU at default clocks might be 95W but once overclocked its more like 150W. All in all 570W with the GPUs at default clocks. But lets face it no one runs a 7970 at default clocks, they go up to about 270W overclocked. Total power usage = 270*2 + 150 = 690.
A thin 120mm radiator will cool about 90W quietly, a thick one will cool the same to 120W. Given that you either need very loud fans OR lots of radiators to cool the total system. But GPUs aren't so temperature sensitive, having them in a "hot" loop isn't a problem, it rarely limits their overclocking headroom. Given that this loop is sufficient (at 2x thick radiator (240W), 2x thin radiator(180W)) to cool the GPUs to just about 12C water delta quietly. If you put a CPU in there that delta rises to 16C which means the CPU will be warmer than if you had put a noctua air cooler on it. 10C is the macimum value you really want to use for watercooling, beyond that performance is a bit rubbish. But for a GPU that is enormously better than the default air and more importantly its very quiet.
So no the CPU not should have been added to the loop, it would overload it at full load and would need considerably faster fans to cool it resulting in a poor volume of fan noise.