Well, you can always have the XT40 replaced by a T54, which bears the same relationship to the T56 that the XT40 bears with the XT38 (i.e. two gas generators feeding power to one combining gearbox; XT40 used two XT38 gas generators and the T54 would've used two of the T56 gas generators). Given the success of the T56, one can assume the T54 would've also done well.