Edelbrock part throttle surge help

Those springs are kinda like the Holley PV.The metering rods have to stay down at idle and very low throttle/load settings,and the rods have to supply the correct amount of fuel while stuck down in the jets for this.
Then at very large throttle/load settings the rods have to be up on the power step, letting max flow occur.
At inbetween settings the spring under the piston has to overcome vacuum and lift the metering rod to a setting that again sets the A/F to what the engine wants.
If the carb was formerly tuned to work with a high vacuum cam, it will have a fairly strong spring that had to overcome the relatively high vacuum. Now with the bigger cam/lower vacuum, that same spring is too powerful for the reduced vacuum and pops up too soon and too high,sending the A/F rich.
But thats a whole different story.The metering rods have to stay down until the engine demands more fuel.
At light throttle/load/cruise settings those rods should be stuck down, and the A/F is set by the clearance between the fat-step and the jet.If youre lean there simply swap to a smaller rod.Unless the A/F goes lean on the powerstep too.Then a larger jet would be called for, to solve both lean problems.
I thought the above article went into that pretty nicely.
Now with the reduced big-cam vacuum idle pull-over gets to be a chore for the engine, actually if you think about it from the angle of airflow;atmospheric pressure isnt pushing as hard into the engine, and so isnt pushing much fuel. So the A/F goes lean.At least until the cam starts making vacuum, which with the 268* is delayed to in-the-neighborhood of 1800rpm.if the timing is right.
So to recap 1) mechanical(make sure the intakes are sealing), and 2) get the timing dialed in,(with more advance comes higher vacuum numbers, which helps stabilize the powerpistons),then 3)tune the A/F. I have done in different orders, which always lead me in circles,and totally wasted my time.Well not totally-totally. I did learn how not to do it.
To answer your last question and marry the others to it; I suspect you need a softer spring to keep the rods down a bit longer, and rods with a smaller idlestep, to provide more lowspeed fuel.Since vacuum is likely reduced all the way to 1800ish and your new cam will make more torque, after that, it will probably want more fuel under power as well. To solve both issues, I would leave the rods in for a bit and instead, go up a size on the MJs .
What I like to do is tune the primaries, by defeating the secondaries.This saves time in the long run. Sometimes the primaries are too fat and confuse the power tuning.Plus with the progressive linkage it can be a tuning handful.