I'd install the H115i (a 280mm-sized radiator) on top as the 300R does support mounting locations on top for a 240mm or 280mm radiator (you can also install a radiator at the rear but only if its 120mm-sized).
I don't think you can install the H115i in front without any modding. The 2x 140mm front intake fans are in between the frame and the detachable front cover, while inside the frame is the space for the HDD cage.
Based on the official 300R
specifications:
Fan Mount Locations
Front: (x2) 120/140mm
Top: (x2) 120/140mm
Rear: (x1) 120mm
Side: (x2) 120/140mm
Radiator Mount Locations
Top: 240/280mm
Rear: 120mm