I used to dabble with the humble pushrodder some 20yrs ago.
we used to stick in Datsun 1800B cranks, gemini pistons/valves, Yamaha valve springs, Duplex double row adj cam cogs and the old Kent cams Cosworth A8 cam grind ! Engine size was about 1800cc .
The reason for that set of blocks holding the rail down is because at many revs(8000+) the std cast blocks tend to flex/break in extreme cases. Roller rockers were/are avail too but we used to replace the springs that separate the rockers with machined bronze tubes to reduce friction .
Bit of overkill for street use though !!