Yes, stock Zenith venturis don't have much wall thickness to modify to larger inside diameters. Best to make from scratch, mindful of where the waist is (13mm down as opposed to 8mm down for Webers).
You might want to change the air correction jet for the Progression circuit. It is 1.6mm for Zeniths and 1.1 for IDAs. The 1.6mm jet will lean the progression circuit richness toward transition.
Paul Abbott
Performance Oriented