Airbus estimates that the A350-1000 retains 70% commonality with the smaller A350-800/900 following changes to the long-range twinjet designed to improve its payload and range.
But it believes customers will accept the divergence when weighed against the benefits of the aircraft, which will be fitted with a higher-thrust Rolls-Royce Trent XWB powerplant.
Speaking to ATI at the Paris air show, A350 programme manager Didier Evrard said customers "all understood" the decision to adapt the aircraft.
Externally, the aircraft will appear little different from its previous iteration. Evrard said the pylon would be reinforced to accommodate the larger XWB engine but would be unchanged in size.
Evrard said that early indications suggest that, although the engine has a larger core, the aerodynamic lines of the nacelle will be "unchanged".
He said the trailing-edge extension on the wing, which will be around 300mm (11.8in) compared with the A350-900, has also been optimised.
"We had to look at the positioning of the [flap] actuators," he said. "It's slightly different from the -900, but good from an aerodynamic point of view."
Air conditioning will need to be expanded in the larger -1000, he added, but "this was foreseen".
The -1000's fuselage will remain an 11-frame extension on the -900 and its main landing gear will still have, as previously designed, a six-wheel bogie.
"We believe commonality on the [-1000] aircraft will be 70% still," said Evrard. "Airlines are certainly interested in this increased capability of the aircraft, the increased thrust. All the airlines I've talked to have underlined this."
He said there were no problems of concern with the composite manufacturing process. "We've made all the panels for the first aircraft, even beyond that. We're very pleased with that."
Evrard pointed out that the panel design concept enables the airframer to "balance tolerances on both sides" of the fuselage. "The indication is that it seems to be going quite well, but I'll be more confident when we've assembled the panels and made the barrel," he said.
This fuselage barrel assembly is the next major step for the programme. Airbus is also preparing to deal with managing the supply-chain production of thousands of smaller components over the next few months.
"This is the challenge we have," Evrard said, admitting that the individual suppliers are "not at the same level".
"Some need a bit more coaching, more development," he added, although he declined to identify specific suppliers.
Airbus is also planning a weight-reduction scheme for the A350-900, the first member of the family, which is due to make its maiden flight at the end of 2012.
A350 chief engineer Gordon McConnell said the airframer would implement a weight-reduction programme on the -900.
"We want to get back to a weight that's consistent with what we said we'd deliver to airlines," he said, although he would not disclose further details of the package beyond stating that it amounted to an "optimisation of the design we have".
Airbus plans the -900 to enter service by the end of 2013 while the changes to the -1000 have pushed back its introduction to 2017. In between the airframer also intends to develop its A350-800, for service in 2016.