Upgrading old Campag groupset with modern parts

How old is the frame, Ed?

It's likely to be either 126 mm or 130 mm rear spacing and, as it's steel, then you'd probably find that 130 mm hubs fit in fine with a bit of encouragement.

...oh and this^

You're looking at 2mm lateral movement per drop-out, which is feck all. I have 135mm mtn hubs in a 130mm spaced alu frame. The fit is tight but I did'nt need to try and bend anything.