I have a 1965 MGB with a std gearbox that would grind when downshifting into 3rd and 2nd. When upshifting into 3rd it would also sometimes grind. I rebuilt the gearbox by taking a donor (trashed) 1966 overdrive gearbox and combining the two to make a OD gearbox with NEW SYCRO RINGS. Used the third motion shaft and OD from the OD unit, and the rest (gears and all) from the std gearbox. All works great with one exception: the third gear still grinds when downshifting and sometimes when upshifting- just like before. Replaced the sycros (new). What could be the issue???? Thank you