T140 sprocket 21 teeth

[Jethro]

New guy here saying hello. I wanted to fit a 21 tooth sprocket to my 1978 T140 and I filed the notches ok but I've got a problem with chain clearance as it hits the casting boss where the gearchange shaft goes across. Anyone had this problem as I thought it was a standard mod? Tempted to grind with Dremel but any advice appreciated. Thanks - J