Ive owned 2 f 100a now and they have both grinded (not a word I know) in reverse. I a, going to try the inch forward before linkage adjustments. Otherwise the 3 speed is an old design and not perfect.

 

Even in my 07 silverado you have to play with the clutch on occasion to get it to go in reverse. It's got the NV3500 5sp. I have ground the gears a time or 2, I can say even my Crewcab and my highboy get a grind every now and then. Your still moving forward some and it goes in just not as smooth as you wanted.
My story telling is over back to work

How new is the clutch? Have you been doing a lot of stop and go? My guess is it is alittle bit of both clutch issues and the fact that it's a 3sp and it takes some playing to shift into reverse