I don't think the Necchi will fit a White treadle without a great deal of modifications. If you look at the base boards at
Sewing Machine Work Stand you will see the space needed for the Necchi (per baseboard "A") as compared to the White (per baseboard "C") The White baseboard will fit the model 11, 41 and 43. From what I can see for the White treadles - they look the same size and shape.
Some other points to consider are the spacing and size of the hinge pins.
Do you have a machine in the treadle? If not, I think I would trace the hole on a large piece of paper, cut out hole, making note of hinge pin location, then slip over the Necchi to see the differences.
Janey - Neat people never make the exciting discoveries I do.