Can you select all gears with the linkage not connected?

Are you sure that the box is compatable as some of the earlier cars had the gate in the gearlever and this was then changed to have a gate in the gearbox. If you have a lever with a gate and then a box with a gate it is a pita to align and get to work properley.

I would also be checking either the rods or cables to ensure that they are fitting correctly.