The 550 rockerbox wear problem depends on the usage more than the mileage.
Quality oil and a gentle right hand will extend the life the rocker shaft bores.
Redlining it and reticent oil changes, could see conditions deteriorate rather quick like.
The lowest effort preventative measure discussed has been to squish a few o-rings between the inspection cap holes and the rocker shaft ends.
The Honda solution was to pin a full length rocker arm shaft in place. The 77 update. Unfortunately the available rocker arm shaft part from 77 does not fit into the valve covers of previous the generation.
To secure the multitude of rocker arm shafts in the early model rockerbox would require a bit of time at the milling machine and drill press.
Repairing worn rockerboxes may be more trouble than it's worth.