IMO you can get up to 5 rays depends on how awesome your filtration is going to be. For that tank size you have i don't think a canister will be enough. try looking at k1 media or pond filters.
Sump filter- large sump that has bio media. Trickle filter/wet-dry. I use wet/drys on all my ray tanks. Example- on my 450 I use a 90 gal wet/dry with about 2000 bio balls and a fx5 canister. Rays = tons of wast. You need a lot of bacteria to eat it up.
Good Luck
The guy who is making the tank is asking me how many mm thick should the glass be. If my tank is 8 feet wide, 4 feet tall and 3 feet in depth, how many mm should the glass be?