Richie, I didn't understand your comment about the nine windows. I thought the Mark IIIs had eight windows, +2 halves equals nine?
What about the Hornby's as donors? When I looked at my own Mark threes I recalled that they don't have flush glazing. Would also be a lot easier to pick up Hornbys as I think they're still on sale in various liveries. Not sure whether the Hornby's or Limas are more prototypically correct bodywise, though.