I am curious why the recommendation to convert the 3310s secondaries to the metering block?

The OP is an admitted novice on carbs. Adding the secondary block does add additional tuning capabilities to the carb, but for someone that does not even know the difference between a 4150 and a 4160, that may not be a good thing.

Besides, the 3310 may be the most popular Holley ever produced and the overwhelming majority have lacked the secondary metering block. Only the very first ones had it.

In my opinion, having a secondary metering block on a 3310 only adds expense and complexity that a beginner tuning a mild engine can't get enough advantages from to justify. Which is why Holley eliminated it many years ago on their most popular entry level model of carb.

One last thought here, weren't the original style of Holley's on the OP's car single inlet carbs? Unlike the dual inlet 3310 style? Point being that they look quite different.