Was there finally a solution for this problem? I'm in the same boat: bought a brand new cinelli giro d'italia (26 mm) handlebar and a pair of vintage nuovo record brake levers. The brake lever clamps are marked campagnolo so seem to be original, but they just won't fit over the end of the handlebar. I might be able to force them a bit so they are on the handlebar, but likely won't be able to get the clamps to where they should be on the handlebar without damaging it.
[EDIT / UPDATE: managed to install the levers by using clamps from a pair of weinmann brake levers. The campagnolo clamps have a fixed diameter which would just could not go over the bar ends... The weinmann brake clamps have a bit more flex: you can play a bit with the two ends of the clamp which are held together by a nut (take out the nut out of one of the ends of the clamp).. It did not go easily but it works.]
Last edited by ARNAERT; 04-21-20 at 12:45 PM.