The 764 has a significant increase in MTOW and MLW compared to the 763, so using its gear and structure gets the needed increase needed. Engineering is then to shrink the length to what is desired. The structure is already certified, except the shrink and the new engines. The pylon from the 748 could possibly be used as it was set up to adapt from and to the same engines.
In all this reengine looks to be straightforward with low risk, the big question is would it add 200+ orders, sufficient to cover the development costs.
Development cost is one of the main reasons it would probably stay an 400 at it's existing MTOW - or a slight bump if the limitation to MTOW was runway performance before - which the extra 3,000 lbs of thrust per side from 2B's would help with (the 400 was offered with 63,500 engines).
As shown earlier in the thread RB211-524g's are within a few pounds of 2B's so the wing should be able to take them without reinforcement, and the thrust bump is the equivalent of taking a narrow body engine from 20,000 to 21,100 lbs of thrust.
As Jay rightly points out the interface for a 2B has probably already been pretty figured out (at least on the plumbing side) you would just have to adapt the shape/bolt pattern for the 767 wing.
Maybe new wingtip treatment and that would be about it. In terms of development cost this should be relatively inexpensive in the realm of programs.
From your comments I can tell you realize the 764 already has a significant MTOW and MLW bump compared to the 763, but other posters do not seem to realize this.
A quick comparison: 763F 764 76XF guessed
Engines (2) CF6-80C2 (2) CF6-80C2 (2) GEnx-2B
Thrust each 61,500 lbf 61,500 lbf 66,500 lbf 8% more thrust
MTOW 412,000 450,000 lbf 450,000 lbf 9% increase
Max Payload 121,000 102,600 lbf 140,000 lbf 19% increase
MLW 326,000 350,000 350,000 lbf 7% increase
This seems to be a very balanced increase where this new freighter is 15% more capable than the current one, the added thrust of the -2Bs cures the prior diss of the 764 - undersized engines. As you noted elsewhere, this is a freighter project, I think it unlikely that a pax version will even be done, but it could. The $2B question, will it sell enough over 20 years to pay off the costs, it would need at least 200 orders to pan out.