Gotcha. Would the EK-AIO 240 be a wise plug and play choice for a 3900x? Or 120mm AIO still better option? I really don't want to mess with modding.
The EK 240 is a decent choice for this case. It fits in both 3 slot and 2 slot configurations, and can fit with two slim 120mm fans. If the radiator end caps end up being 120mm wide or less, you can push the radiator to the very front of the case and use one slim 120mm fan and a full sized 120mm fan to give you better performance. If the end caps are greater than 120mm wide, then you can sand them down to make them fit, but this will most likely void your warranty.
Another 240mm AIO option is the Corsair H100i Pro XT, which allows for one slim fan and one full sized fan. Thing is, it can be slightly too thick, which means when you put the top panel on, it pushes the radiator down, and thus pushes the fan below the radiator into the motherboard, preventing it from spinning. This can be fixed by using washers though.
120mm AIOs are usually the best all around option, as they give you the best balance of GPU and CPU thermals. With a 240mm radiator, the GPU temps will be warmer because you don't have a dedicated fan exhausting it's hot air like you would if you run a 120mm radiator. 120mm AIOs are also the most compatible, as this case was made to accommodate most 120mm radiators.
Since you are using a 3900x, I would go with a 240mm radiator and a 2.5 slot or greater GPU.
If you want to look more into this, you can look at my posts on reddit and go through all the comments where I answer questions and give more detail into the builds.
This one is the one I am currently using with the Corsair H100i Pro XT, and
this one is the one I was using with a 120mm AIO.