There are a few threads on Rennlist and Pelican talking about getting the gearbox apart after this happens, and I guess it comes down to how much time do you want to spend vs how many dollars you want to spend. The fastest and cheapest solution may be to get another low mileage used box, assuming you know the condition of the bearings and synchros.
I would try to remove the shift rod, and if you can't get it out because the 3/4 shift fork won't go back into neutral, then look for another one, but I say that because I have a parts car with what I hope is a good transmission.