Dave, I don't see how the 23mm height Intellect Lipo pack cannot fit. NiMH cells are 23mm in diameter. From the pics I see around the web, it looks like the cells mounted in the 511 are clearly lower than the top deck/rear of center bulkhead area.
http://www.redrc.net/wp-content/uplo...yatrf511-3.jpg
I do, however, see how the lipo pack may come in contact with that center bulkhead, but I would think you can slowly and carefully Dremel the lipo casing to possibly make it fit, or Dremel off the end screw hole on the bulkhead and only use 2 screws to mount the rear top deck to the center bulkhead.