First you have to pull the transmission and take a look at the damage to the front case, if it is good then you can use any of the gasser transmissions Mike mentioned above like yours(longshaft), all you do is put your front case onto the doner transmission.
When 5th went away I would imagine the whole truck shook/jerked/made a loud noise, that was likely the cluster gear/counter shaft self destructing, it should have been making a whining noise or some other kind of noise before that except in 4th gear, if it makes a noise at an idle in neutral it is time to fix it.
You need to check your front case first, if it is not busted between the bearings or the hole wasted, you are good to go with the case, the front cover might be an issue if yours is broke unless you find a transmission(pre 1983) with the small bearing, you can then use that bearing cover.
If the hole is buggered up but not broken between the bearings you can likely have the lower hole turned out to the large 63mm bearing to save your front case.
These diesel transmissions are getting very hard to find, everyone wants them to fix their diesel or they use them to put a Zcar transmission in there Roadster.