Did you happen to use a new oil drain plug ? we've seen longer aftermarket plugs interfere with the gears
is the shift pawl and shift star in correct ?
I'd pull the clutch cover and clutch pack if needed to see the shift shaft/pawl in action up/down thru the gears, fix/replace if needed, spinning the front drive gear will help it shift when not running, then ......
you should have spun the front gear and tested shifting thru the gears before finalizing the rebuild, and you'll most likely have to split the cases again