Great to hear that your Gymnarchus niloticus is doing well.
I still have mine and it will be 14 years on feb 17 2014.
Mine never grew fast after it hit about 60cm (24") and is only about 94-96cm (37 to 38 inches) imo. friends think maybe 101cm 40".
I found mine to be very curious and bored in small tanks. I had it by itself, mostly in 1814 litre and 2116 litre tanks this year until september and it also rearranged everything and tore apart the Marineland 560 and Fluval fx5 tubes and strainers, non stop attacked the heaters (metal). I gave up on gravel.
My daughter and friends Dave and Kevin sectioned off the narrow end of the 56k litre for me during July and August and it is now housed in an area about 215cm by 185cm by about 183cm deep.
The aba is very active in it, eating better and although it rearranges a circulating pump daily it no longer damages tubes and strainers as I have a swimming pool drain in the floor connected to a bead filter. I still have it's metre long 15 cm wide den tube in with the aba. And the tank is served by the same large tankless natural gas hot water heater that heats my larger 196k litre tank. As a back up I have an oil tank hot water heater in case I have problems with the natural gas one. Both are connected to a back up natural gas generator. I had an electric gas generator as a back up but that only operated for 14 hours on a tank of gas. The natural gas generator can run unlimited.
The mesh separating the aba from the 80 plus fish in the now reduced main section allows the aba some interaction with other fish without killing most of them. It really reduces the aba's boredom.
While they sectioned off the aba's area they also sectioned off the L in the 56k for my Neoceratodus forsteri. The AUL now has area about 285cm by 305cm by 183 cm deep. The AUL is a lot more active now. It also likes to go to the mesh and interact with some of the fishes swimming in the main section of the tank. It does have a few tankmates it's section. One african tiger fish about 30cm, several Mylossoma aureum, one female Potamotrygon reticulatus and my old 22 year old Hemisynodontis membranaceus. It also has a window on one side of tank which faces outside and often sits on a ledge appearing to interact with my two dogs who are looking at it.
I'm planning a garage build and then a large concrete tank. L shaped, huge, just like your tanks. Possible IM's in your future to get details on what works and what doesn't for very large setups 
