I've been looking at a headset in the FS section that has a 27.0mm crown race. The fork I would put it on is sized for a 26.4mm crown race. I would be using 25 5/32" loose ball bearings in each cup. For the sake of argument, let's assume the bearing cups of the headset fit the frame fine.

Could I theoretically use any 26.4mm crown race with the loose ball setup? If not, why?