I had this problem with my 77 and my 75 trucks. In both cases, it was because the sleeve on the front of the transmission was dry. This is the sleeve that the throwout bearing rides on and when the grease wears off, the clutch gets very hard to push. But, if you take the pressure off of the bearing, you can move it around on the sleeve easily.

There are 2 ways to fix it. One is to remove the tranny and grease the sleeve again. I think most throwout bearings have a grease fitting on them but you can never get to that fitting when the tranny is installed.

The 2nd way is to use a long Q-tip dipped in grease and grease the sleeve by removing the clutch fork boot an slipping the Q-tip in next to the fork. It takes a few rounds of greasing the sleeve as best you can and then having somebody work the clutch. To grease it this way, you need a flashlight to shine in the shift fork hole so that you can see the sleeve. I have used this method successfully because I got tired of removing transmissions.