If using a 27mm crown race on the fork, with a 26.4mm bottom cup increases the stack height much, you would have the option of fine-tuning the ball-bearing size, if it comes to that. Given that the end setup is an angular contact bearing set up for mostly axial thrust, the ~36-45° contact angles should mean that the 0.6mm increase in crown race diameter will increase the stack height by less than 0.6mm (about 0.4mm). About 1/64th of an inch. This is, of course, assuming that all other dimensions for the bearings are the same. I'm assuming that you would use only the crown race on the fork, and that the upper race and cup will both be new (that is, the 26.4mm).

I may not be visualizing what you are doing correctly, though.