http://fiedlerh.home.att.net/images/ST10man.pdf
You may or may not have snap rings on either side of speedo drive. If not, simply measure and record the existing distance from the end of the rear main shaft so you can press her back in the right spot. Everything aft of the third gear clutch slides off the back of the shaft so the speedo drive gear must come off. The snap ring for the rear bearing (inside of case but cant be seen or removed until retainer/rear shaft assembly is moved rearward) will have to be released into the recess in the retainer and then the retainer slides off of the bearing.
Also, depending on gearing of the tranny you may have to slide the rear bearing and bearing retainer backwards on the shaft to get 3rd gear to clear the second gear from the front on the cluster gear. Yes, even if the cluster is laying in the bottom of the case. After you gain this clearence, then the rear main shaft can leave out the back of the case. Front main shaft comes out through the inside of the case also after you remove the snap ring on front of bearing.
It is all pretty routine 4 speed stuff but holler if you have any questions