have already found a 240 gal, 8ft long, i think that should suffice for a Cortez ray. i don't see the lighting being "too much" for the ray, but I can't find much documentation on it either.
I would gather from observation, and the recommendation from the CMA that the tank would need to be a minimum of 3 feet deep for comfortable turning. An 8ft 240 gallon tank, is probably 8x2x2, but I could be wrong. Another quote:
"The practical implications of the sharks' mode of transport are that they need a lot of room, they can't change direction or level easily, and the "hate" square-system corners".
IMO, I don't think standard systems that hobbyists employ are really sufficient for these animals. Have you ever seen sharks or rays kept well/long term in a square or rectangular shaped aquarium? I have not. Not to say it can't be done, I am not an expert, but nearly all enclosures I have seen of the volume you are even remotely capable of providing are almost always constructed of concrete. The tanks are set up like pools, shallow and wide. They meander and have a very organic shape that conforms to the way sharks and rays turn. They are nothing like a small rectangle. A Circle would probably be the most suitable standard small shape, but again, I am not an expert.
Finally even though the CMA has some information on sharks, it is a whopping 8 pages long, and although Bob Fenner seems knowledgeable on the subject I doubt even he would want you to use his book as a guide to keeping sharks and rays. I quote from here as I own 17 reek keeping books, only one has any information on sharks/rays. It is a specialized topic for sure. You will need some real information on keeping sharks and rays, and most likely a custom tank to house them I would think. There are some books out there, but as I have never read them, or kept sharks or rays I can't reasonably recommend one.
This is off topic from LTAs and anemones but as I am not tracking your posts elsewhere I don't know if you are asking the right questions to the right people. Unfortunately I am not the right person, I can only tell you what is in one book I own.
On the positive side you have made improvements to your current setup, by adding sand, and appropriate lighting. You are on your way to having a decent "reef" setup. Certainly one capable of maintaining a LTA and clownfish. Back on topic, I would think for your current tank aside from your disease/fish compatibility issues, your filtration/flow would be the next thing to look at.