1st prize for guessing where this image came from.....
According to multiple sources the Longtail version of the 720s is due in 2019 and mclaren have announced it on their instagram, The name 780LT is a guess and all the sources that i have trawled through (and believe me there were alot of sources!) haven't given me anything but speculation, all the concrete evidence that I Have is that a 720s LT model is coming. as Motor1 ably states "The British supercar maker has confirmed that an LT, or Longtail, version is in the works – and it could come with even more power"
"we are family!" all the modern LT models
The Mechanical bits.
According to the 2017 Top gear supercars magazine, Mclaren bosses have said that the 720s's chassis could handle up to 800BHP so it's definitely going to be epicly fast. I'm guessing that they will deploy the same gear box that is on the 600LT. As CEO Mike Flewitt said "The 675LT was a huge success," said Flewitt. "We’ll certainly repeat Longtail, and now that the LT brand is established, if we announce an LT derivative of a car, then everyone will know what to expect." so naturally lightning fast gear changes, all round lightness and much carbon fibre. Imgoing to go for around 750BHP and about 790 N.M of tourge