I have a ally rad I got cheap too, says it fits a 5th gen and thought I'd get it to fit. I will, quite easily, but I'm going custom with alot of things, including fitting new slim fans. HOWEVER...this rad is smaller overall dimension to the one I had in my 4th gen. It's thicker core so overall more cooling potential.
Could you measure the dimensions of yours as a whole unit, then measure the distance between the bottom leg mounts and same for top please. Plus maybe a pic of the rear to show the fan mount points. I'll do the same on mine, if it turns out you have a 4th gen one and I have a 5th gen one, could be a potential swap situation to make our lives easier?
Let me know. I'll measure mine tomorrow straight after work.
Cheers,
Rob