Perhaps a very light smear of lithium grease on the nipple of the input shaft? In 2015 when I was removing and installing the transmission in my '65, because of rebuilding the one, installing the spare, needing to tear apart the the rebuilt because of issues, and reinstalling the spare, it wouldn't go into reverse, and first was grinding like you mention. I unbolted the trans, put a very small bad of lithium grease on the input shaft nipple, and no more issues going into gear. I later found out the pilot bushings are oil-impregnated, so I guess it may have just been a little surface rust or something on the nipple. Either way, that may be your problem, and a whole lot easier than removing the clutch if you don't need to. Start with the easy stuff and move up to the more difficult stuff.
