4spd shifting difficulties

If you're sure that the shift rods from the shifter to the transmission are adjusted correctly then the other problem might be that the clutch is not adjusted correctly. When the clutch disc (friction plate) gets hot it will grow in thickness, .010"-.015", and therefore if the clutch pressure plate does not fully disengage, when you push the pedal to the floor, it will make it hard to shift. How far do you push the clutch pedal down before you can feel the clutch starting to activate (called freeplay). The other problem that could exist is that there should be a bronze bushing pressed into the end of the crankshaft that the noze of the transmission input shaft fits into. If the clearance between the I.D. of the bushing and the O.D. of the input shaft is to tight the input shaft will remain connected to the crankshaft (friction) even when the clutch is fully disengaged.
There is a third possibility but I don't think temperature would necessarily have a part to play in this one. One of the engine crank bearings doubles as a thrust bearing for the crank. When you depress the clutch pedal the clutch throw out bearing not only pushes on the pressure plate to disengage the clutch, it also is pushing on the crankshaft and pushes the crank up against the crank thrust bearing. A worn thrust bearing will cause you to loose some of the travel needed to disengage the clutch.