Geared setup for 126mm rear spacing

Thanks for all the info.

DO NOT attempt to change the spacing. The rear stays in a 731 tubeset are heat treated 753 and cannot be cold-set without weakening or breaking them.

You won't want to cold set it, but it's easy enough to jam a 130mm hub into a 126mm frame, and that's what I'd do in your position.

I wasn't planning on going for the cold-set option, just pulling the dropouts a couple of mm apart to fit a modern wheel in (i.e. so that when I remove the wheel the dropouts return to their resting position), as mdcctester suggests. Think that's what I'll do if it won't affect the integrity of the steel, as it sounds like the price you pay for STIs.