A-833 Stuck in gear

The only thing that is interlocked is the two levers for forward speeds. So if you are certain that you have 3/4 in neutral, then it's time to pull the side cover. Maybe you didn't get the fork in, maybe it's bent, or maybe the side cover has a problem with the interlock.

In fact, when I used to install a gearbox, I would purposely put the box into reverse and a forward speed simultaneously. This locks the shaft to the box, so that you can "wiggle" the gearbox and get the splines to engage.