You need to check and make sure the inner teeth on the synchronizer are not worn or rounded off . A new balk ring won't fix it if thats the case. Its the friction between the brass ring and the outer cone of the gear that slows it and times it to lock into the synchronizer without grinding. Very close tolerances are necessary for it to work right. Are you sure that the thrust washers are fitted correctly? With the cover off 3rd and 2nd gear should be exactly in line with their counterparts on the lay gear. If not , find and correct. Your gearchange is made with inertia , too much travel and the synchro can be moving with too much force to stop the gear from spinning and cause grinding , if the parts are less than perfect or if you have a thrust washer issue.