If you’re pulling the synchros out you might as well get them bead blasted and put them back in. This will be your issue - the synchros are worn out and smooth so they no longer grab to match the gearset speeds with the engagement hub.
Rough them up and they should work properly again.
Gearbox work can get tricky if you don’t know what you’re doing - but as always, the best way is to learn! Just have a back up plan if the rebuild doesn’t go as planned.
Cheers
Bennie