"Very often" is every single time. It's in the PBS shifter instructions. The reverse fork must be trimmed, best done with a cutoff wheel. The nosecone(tailcone in our case) must be removed to get to the fork. I put a plastic bag over my trans and poked the reverse fork through. A Sharpie mark is a good guide for where to cut. It isn't much to cut, but it must be done.
Temporarily stick a drill bit into the tapped hole at the bottom of the diagram and rest a straight edge against the drill bit, make your mark and cut. Easy peasy.
That picture is all I can find, the PBS website is down.
I usually use two hands to get reverse, the lockout spring is STRONG.