I think the AX1500i is bigger than the AX1200i, which I use at the bottom of my 900D. I have put a 240mm rad on the same side as the PSU so in your case, it will be a tight fit. I would look around and search to see anyone with a AX1500i and a custom water cooling system in a 900D.
As for the radiators, I use a SR1 480 on the none PSU side with push only 25mm fans. Still have room in between but it would be too tight for my comfort and air flow if I were to put a push and pull configuration.
Here's a
thread that should answer most if not all your questions on measurements.