I would cut the overflow out, patch the hole, and move it to a corner. Would be much better suited. Still though 36" wide is not ideal for adult black rays. Females get 24" plus. I'm only assuming your going black rays because if your profile picture.
You will be fine for quite some time as it takes a significant amount of time to reach those high sizes but of course will eventually need upgraded. As for the middle overflow I also think you are fine, at worst you are 4-6 inches in at one small point and the majority of the tank is the 36" wide. height is fine.
Not to mention how awesome it is to watch 2 rays try to breed in a tank they can't even climb the wall of cause it's too short. Can you say clean up in isle living room?
Maybe they should have posted a pic of said tank with HUGE overflow right smack dab in the MIDDLE of the tank. Not sure where the 4-6" comes form but the overflow looks to be at least a square foot right dead center in the middle of the floor of the tank.
Clearly what some consider a bad idea is just "fine" for others...... We obviously have many "levels" of ray keepers here if opinions vary this much.....
Too small for adults. A black female might find that uncomfortable under 2yrs, and they can use a lot of real estate. I haven't seen the tank in question though... speculating based on other tanks with similar builds.