I have built a custom WC in my 300R, with a few simple mods. I think with this you will be able to have at least 2 WC blocks, CPU & GPU, with just using on the front radiators. My setup before this was a front EX-240, to CPU block, to reservoir, to pump, to radiator. Very simple. Wanted to get more complex, since I added a second GPU and another radiator.
MOBO: ASUS Crosshair Formula Z
HDD: WD Black 1TB
SSD: Samsung 830 128GB
RAM: Corsair Vengeance LP
GPU: 2x MSI TF3 7950
PSU: Corsair AX1200i
CPU WC Block: XSPC Raystorm (AMD)
Front Radiator: XSPC EX-240
Top Radiator: XSPC EX-280
Pump: Swiftech MCP655-B
Reservoir: Swiftech MicroRes V2
Pump Top: Bitspower BP-D5TOPP2-BK D5 Mod Pump Top V2
Fittings: A couple of angle fittings to make everything fit.
How to get it all to fit:
Front Radiators: I drilled out the rivets for the front HDD cage. The EX-240 will fit easily without drilling any extra holes. The radiator has to have the G1/4" threads on the top for this to work. From the front fan pictures you can see 2 brass screws. I had to go to HomeDepot and look for longer screws. I forget the what the screw length was (if anybody is interested I can look for the extra ones and measure them. The length fits perfectly, nothing is touching the fins. They basically go through the fan, the case fan mount and into the upper fan mount of where the lower fan should be. I circled it on the EX-240 pictures, but think of it as being on the other side, since you want the G1/4" threads facing towards the MOBO not towards the front. It is very stable even with just two screws. You will scratch the paint on the radiator. it took some trial and error.
My basic way of doing it was, putting one of the brass screw all the way through the fan and case screw hole. Then I would screw it in a bit more so I can line up the radiator fan mount. I would then press the radiator fan hole up against the screw and continue screwing it in only for a couple of turns. This keeps a hold of the radiator while you properly screw the other screw in. Level the radiator and push the radiator as far against the front fan mounts. With the radiator level, you should be able to screw in the 2nd brass screw, through all the holes; fan. case, and into the radiator. Now unscrew the first brass and repeat the steps on how you got the 2nd screw in. This will ensure a very tight fit against the case fan mounts.
I can make a video if anybody wants.
Top Radiator: This is pretty much a passive radiator. I can't fit any fans on it. But the EX-280 I have on it now is a bit long, and does hit a little bump, where the mesh is attached to the case. It doesn't really have a big impact on it. I had to cut a part of the 5.25" drive bay bottom to be able to fit the tubing through. I guess I don't really need this if I have just CPU WC. I was trying to get my GPU WC too. Now I don't know if I will be able to in this case.
I had an EX-240 there before, with only the front fan. Not enough space for 2 fans on the EX-240. The EX-240 will fit perfectly, depending on your MOBO. I also didn't have to take out a piece of the 5.25" bay bottom, since the radiator was shorter. All it took was some angle fittings.
Reservoir: Mounted it using the vent holes along side of the PCI slots.
HDD: I have on top of the PSU
SSD: Pretty much between the pump and radiator.
Cable Management: I think the 300R was not meant to take this much wiring in the back. There is not enough space and clearance. I have 1x MOBO 24pin, 2x CPU 8 pin, 4x PCIe 6 pin, 1x Molex. It is just too much for the backside cover to handle. I think it will all fit if I had only 1 GPU.
If you have any questions, feel free to ask.