Just a thought: drill an exploratory hole horizontally into the pushrod relief to gauge the wall thickness. If you're lucky, it's big. When done, tap & seal it with a long screw.
This would mean that enlarging the hole (toward the fulcrum, not as a circle) will allow greater latitude in where the rocker's pushrod end must locate.
Yes, it will introduce a slight angle to the tappet cup, harmless (and normal in many engines).
