Sounds like you've done a good job crazydane and paid attention to most of the details. I'm betting the synchro keys are the problem. Long ones where the short ones are supposed to be, thus pushing the brass synchro ring to tightly against the gear it supposed to mesh with.
DO let us know what you find out!
Vacuum cleaner? Well theres a new one on me, I used a magnet.
I also used Vaseline to hold those various nasty little roller bearings in place, but it would melt and the bearings would fall out.
So I put some parts in the freezer to cool them down and 'worked fast'. Next time I'll get some of that 'trans assembly lube' I've heard about, maybe it's got a higher melting point?
I recently finished an Austin Healey trans and overdrive unit and the book (an old 1962 edition) actually recommended automotive GREASE for assembly, an idea which I quickly rejected. Concern being the grease won't mix well, or at all, with the trans
oil. In comparison to a top loader that Healey trans was a nightmare, they must have used children with really small hands to install the dang counter shaft gear. Impossible I tell you!