Sorry if my previous posts came off as flaming. In answer to your question, I guess I'd agree with the others and get an FX5. My only concern is that the power of a filter like that will create some crazy currents in a 55g. This probably applies to most filters that are large enough to keep a 55 clean enough to keep a ray, which is why I really don't recommend that you keep a ray in a 55g tank. But that point has been made, and I'm glad you have a big tank on the ready.
However, I would reiterate that you really, really shouldn't be taking a ray so soon after an apparent crash. Any ammonia is bad. And ammo-lock is NOT a solution. If your filter is still recycling the worst thing you could do is throw in a bunch of extra bio-load. And any big filter you get will take some time to really get established. Is there any way that you can delay receipt of the ray?