There are only so many things that can cause a transmission to grind. You just have to find out if it's caused by something internal (such as bad synchros) or something external (clutch not disengaging, bad shifter, etc.).
If the pressure plate wasn't disengaging the clutch, then you likely wouldn't be able to put it into gear with the motor running. The clutch and input shaft need to spin freely in order for the synchros to function properly. If the clutch is still spinning with the motor, then the transmission input shaft and gear cluster will be spinning with the motor too. Grinding happens when there is an RPM difference between the dog teeth on the side of the gear, and the collar that's trying to slide over those teeth. A seized pilot bearing can have a similar effect because it causes the input shaft to spin with the motor even when the clutch is disengaged.
Now.. some questions...
-When did it start grinding?
-Is the grinding getting worse/changing?
-Does it go into first easily?
-Are you SURE the shifter is FULLY engaging the gear?
-How does it act when shifting quickly/firmly and also normally at low RPM's?
-How does it act when shifting quickly/firmly and also normally higher RPM's?
Try to keep things simple. If you're only having problems under certain conditions, then what about those conditions would cause the problems you're having? If you can drive around town normally with no issues, but it grinds going into 2nd at redline, then your synchros probably aren't grabbing like they should. Issues like a bad pressure plate or shifter might be more apparent all the time or in other conditions.
Anyways, just food for thought
Good luck on figuring this all out.