First off, well done on getting your dad on board!
I have to admit, that I personally wouldn't go for any more than two pairs, three at a push. Because, I had a 180 which was run by a pair of convicts, I know that the proposed tank is larger, but if you had two pairs at either end you will lose 2 or 3 foot from both ends. Meaning the fish will have to reside in the centre 4-6 feet. So if there was an extra pair there would be huge fights. I know thats a very simplistic way of looking at things but trust me it could happen, especially with the larger cichlids. If there was no pairs involved you would be okay with a heavier stocking. But it is less about volume and more about floor space with cichlids. The 31" height is no use to anybody IMO, unless you have fish which are tall bodied. I view anything over 24" a waste for cichlids. Just my opinion.
So really you need to go for stocking a 390 not a 480.
Finally you say you want to see them defending their fry as they would in the wild. Well... A JD would never bedefending its fry from an oscar for instance - because they don't even live close. If I were you I would look into a biotope, then youwould get a real indication of the wild. Just a thought!