Depends how you modified the airbox, I'd say you should be fine with 70-80 mainjet, 90 should be ok to start with.
I was running modified stock airbox (total airbox inlet diameter was 26mm) and due plus/giannelli shot setup with 21mm carb with 76 mainjet. I am sure it wasn't lean because i did more than 10 000 km with it.
It was horizontal minarelli engine.