Have you pulled your shift lever off to see if the finger at the bottom of the shift lever is broken?
You can use a longshaft transmission from a gas truck, but you have to put the front diesel case on it.
If your shift lever is good, your going to have to pull the transmission anyway, so pull it apart and find out what is wrong with it.
I am not a writer, but I wrote this article about taking the front case off one of these transmissions and putting another case on, it will work for you also.
http://www.nwdatsuns.com/viewtopic.php?f=35&t=72