To come back to the 737;
BOAC, do you have a reference for the AD you are discussing? And what is the mod? And what effect does it have?
Presumably the point of fiddling with cross-feed and single centre tank pump operations is to ensure a Master Caution is generated when that pump runs dry; if both pumps are on then one may run dry for a considerable period, with no MC until the other does too.
But flying the Classic we only have restrictions on operating the centre tank fuel pumps on the ground.??