All of my toys in the '60s and '70s were GM fourspeeds. Two things come to mind....

Is the clutch releasing all the way? If it's dragging a little, they will act that way. You might try adjusting the clutch linkage push the throwout bearing a little farther in, to make sure.

There are some brass rings that slow or speed up the gears so they will get to the same speed before they mesh. If the synchro rings are worn, it will do that.