[jhv48]

A T-5 holds 2.8 quarts. Without knowing what was in there originally, I would drain it and refill with the fluid of your choice as recommended by Tremec.
But first, loosen the fill plug and see if it’s full to just below the threads. If full, then you’ll have to look elsewhere.
Usually, if your clutch is dragging, getting into reverse will always grind. So if reverse is ok, probably not the clutch.