If your car is dead stopped, and the clutch is all the way in, and it still grinds trying to get into reverse, it is not a transmission problem.
The input shaft is supposed to be stopped in order to engage reverse (without a synchronizer). If your clutch freeplay is right, the pressure plate should completely let go of the clutch disc, allowing it to stop. If you have grinding going into reverse, the input shaft/clutch disc is still turning.
Some causes:
clutch not releasing
pilot bearing in gland nut buggered, causing the pilot bearing to drive the input shaft.
splines on the input shaft for the clutch disc are rusty and not lubed. This prevents the disc from releasing away from the flywheel
broken moorings of the clutch tube in the tunnel. If the clutch tube is flopping around when you push the clutch, the pedal's travel isn't transmitted fully to the arm on the trans.