Previous posting discussions eventually "boil down" to the type of crankcase ventilation you want to have on your engine.
Recap: Some 216 valve covers had slots (possible same issue with water from vent). Some 216 valve covers did not have slots but had an oil fill cap breather. 1954-55 1st 235 and 261 valve covers had slits except those that had RPO 217 (early PCV)....slits would negate the purpose of the PCV. 1955 2nd and later had no slots or slits but used a oil cap breather except RPO 217. 1958-62 cars had the center position fill cap (low profile hoods) but trucks continues with the fill cap at the front.
Add to the mania mix the 1964 California Emissions act of PCV and the need to retrofit cars/trucks back to 1950.
"Adding CFM to a truck will only help at engine speeds you don't want to use." "I found there was nothing to gain beyond 400 CFM."
