Clutch fork won't clip back in.

Ok, so I've measured my old original T/O bearing against the new one and they are the same both in overall length and between where the fork clips on and to the roller bearing surface where it meets the fingers in the clutch.
So I've ruled this out as a cause. The bolts area all tight on the bell, box and clutch assembly.the gearbox is brand new 18 spline from brewers.
This leave me with finger height. If I can decrease the finger height, I think it will fit but I've never done that before . I can see 3 bolts that fix the fingers on but I'm guessing this should be done with the box out and the clutch disc, flywheel and pressure plate bolted together? ?