They did mix up pushrod length for a stud mounted rocker with an article on shaft mounted rockers which adds to the confusion. I'm not sure about cutting the Hughes rockers, I would not have done that. That cam didn't seem radical enough to require cutting the rocker arms. A stout beehive or the dual conical spring or a smaller spring of some type most likely would've fit. I'm also not a fan of adding the shims under the shafts. I've worked on a bunch of race engines over the years and I've never had to add shims under the shaft. So when I see that I wonder what is going on.
What I thought was interesting was the root problem. If someone came on here and said their engine was breaking rocker arms how many people would ask if the pistons were installed in the correct holes? That is what the root cause was but it isn't something I'd normally put on the top of the list of problems.
The beehive spring would give you some rocker clearance but it still doesn't fix the geometry issue.
As you say, that spring wasn't big enough to be a problem. The problem is the shaft is too low, and too close to the valve.
Move it up and away from the valve and any retainer would clear.
