the top and bottom of your centre board case openings sound as if they have been at some time "tailored" to fit firmly to two centre boards that were slightly different from port to starboard, and that probably the "thinner one of the two has since been replaced with a "standard" one, and therefore the new one now won't fit in the same case opening. If this appears to be the case then ease the centre board into the top of the case, mark with pencil where the tight spots are, remove the c/board and gently file back those areas, testing very regularely with the board for fit, (always dust off the area that you have worked before trying the board and make sure that the board doesn't build up residule on its surface) do this untill the board slides into the top opening. Repeat the process then with the bottom outlet. If there is absolutely no signs at all (either "bog" or "gel coat" build up) that the openings have been "tailored" try ordering a new "slimmer" board for the tighter side. the manufacturer can make a board slimmer to a small degree, when they join the split moulds after the layup of the new centre board. Or you can combine, as described above, with wet rubbing back your board as well, but you will only be able to take off a very small amount from the board before you sand through the "gel coat" and expose the fibreglass in the laminate (which is not advisable) If you sand back the case openings and get the required fit, just finish the top and bottom openings with fine wet and dry paper to about 800 grit.