Here's what I see.....
Firts off you are using too MUCH silicone... Use a thin layer, put some uner the bushings and on the top of the bushings, (thin layer) the access that gets squeezed out can get inside and creat issues you won't like. Also the two silver bushings get rid of them or you will not like the end result either, they will seize whne they get hot regardless the amount of lube... NOW to get to you problem it appears to me from the bottom picture you have too much space or more than the rest between 4th and 5th gear on the shift key shaft. Is this so. Should not be, also whne putting the gears on that shaft there should be a beveled washer then first spur gear then washer then gear then washer then gear. With bevel side going toward shift keys then after the 5th gear a thrut washer and bushing. Also make sure that the shift shaft has no burs on the shaft on the lip of the shift key groove. But from what I see the spacing is off somehow it don't look right between 4th and 5th.
