Question for Holley carb gurus
secondaries blades were almost completely closed and not exposing ANY of the idle slot
That is correct.
secondary throttle blades to only show a "square" of the idle slot.
And this is not. The secondary t-slots are located further up the bore than the primary t-slots. The main purpose of the secondary idle system is to help when the secondaries are just a little open. They're not supposed to be contributing fuel when driving around town.
I believe the carb model is a 3310.
It will be stamped on the choke tower. There will be the list number - revision. There may also be a date code.
Do this while the carb is off. Take some drill bits or feeler gages and measure how many turns of the idle speed screw it takes to reveal .020 to .040" of the transition slots. Start with the throttles completely closed, screw tip just touching. Write that down some place you can find it when you need it. Then you'll always know how much t-slot is exposed.
I wouldn't go through the trouble of switching the secondary idle screws on a vac secondary unless its really needed.
If you want to crack it a hair more open, do so while the carb is off. It as actually possible to adjust while on the intake - sometimes - but not easy.
Regardless, try another degree or two of timing at the same rpm as before. Say it was at 13* at 600 rpm, then try 15* reduce throttle stop screw to get near 600 rpm. Readjust mix screws by turning them in until the rpms just start to drop, then turn them back richer 1/8 to 1/4 turn. Then check engine in gear. Repeat with 2* more if you want to see if that helps more or hurts.
When my 3310-3 was fairly new, I reduced the throttle position at idle the 'old school' way - drilled holes in the throttle blades. Then had to reduce the IABs (with wire). Its not wrong. I'd do it again in the right circumstances. But in most situations I would start with more initial timing, then maybe crack the secondaries a little, and if yet more air was needed at idle then consider drilling.