Missing 3rd on 904 trans

Is your shifter;
P-R-N-D-2-1
Is manual low all the way to the back/bottom?
and can you feel each notch as you pass into/thru it?
If yur not sure,
put the cabin-shifter into Low, then get underneath the car and remove the shift linkage (the KD can stay) from the shift lever, which should be all the way to the back. Push the shift lever all the way to the back of the car, and tug on the linkage to be sure it is still in Manual-Low, then attempt to drop the link into the shift lever. If it does, then. lift the link out, and push the lever all the way to the front, followed by the shift-rod. You may have to get out from under there, go back into the cab, put the cabin shifter into Park, and then climb back under it.
Verify that the trans is in park by attempting to rotate the driveshaft in it's normal direction which is CW from the front of the car.
Now return to the trans and attempt to drop the linkrod into the hole in the lever.
If it does line up, and the linkrod drops right in; then the parts are matched, and the cabin-shifter is synchronized to the trans..
If it almost lines up, then you may have to tweak the length of the linkrod. You will have to find the adjuster and make it happen. It's not faraway, and you can usually get to it from underneath.
If it does not line up at all, then you have the wrong lever on the 904. Find and install the right lever, Synchronize it ......... Then roadtest it again