Flat screen does have to be as big as standard backbox - no reason why the boards cant be housed in the cabinet - all those looms, so bloody dated!
You would probably need a 4:3 aspect ratio flat screen instead of a wide screen, otherwise the head box would look small.
And having the boards mounted under the playfield would make it more difficult to service - leaning over the cabinet to get inside is a bit of a pain, and the Pinball 2000's have the driver board under the playfield which is difficult to get to and pull out without removing the playfield itself.
