The rear triangle should be 126 OLD but it's no big deal inserting a 126 OLD wheel into a frame set at 123 OLD. In fact, some bikes built around 5 speeds were set at 123 so that you could run either a 120 or an 126 OLD wheel.

Also there are cages that take 11 1/4 inch ball bearings. I like using them on the fixed cup side and running loose on the adjustable which makes installation a bit easier than simply running loose.