Boeing would not 're-design' a 74 or 77 out of 100% composites. The design is firm (in the case of the 74, it's over 40 years old, actually).
Boeing is sweating over the idea of having to build a larger wingbox to accommodate a 6 wheel main gear array and beefing up the fuselage of the 787 (the Airbus 350X is being designed to take on both the 78 and the 77. If the 78 is ever extended to the point where it would could go head to head with the larger 350X's, it'd likely need to go from a 4 wheel truck to a 6 wheel truck, necessitating the changes mentioned above, as well as others). Boeing doesn't want to build a larger 78 b/c it would cannibalize 77 sales. They also don't want to spend the $ to pay for the design and manufacturing changes that the larger wingbox would require.
The point of all of this? If Boeing thinks it's likely too expensive to design and build the larger wingbox on the 78 (a plane that hasn't even really flown yet), you can get a grasp of how much $ it would cost to re-design the 74 or 77 in composites.
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