I don't completely agree on what the guy from AED said to you on the the reason why the primary holes are drilled. Usually that is for engines that are so high up there in cam size that its normally for something that is not a street cruiser and does not spend much time with very little throttle opening.
I have always had to tune with thee secondary side being adjusted regardless if the front has holes or not. One time I had to drill holes but that was because I did not have a rich enough fuel curve on the idle circuit and had to open up the primary butterflies and secondary side too much and the mixture screws would not work.
After re calibrating my idle circuit by bigger idle feed restrictors and stuff I replaced the drilled butterflies and fixed the base plate back with new plates and the thing would then adjust and work like a charm. Just a lesson to be learned. In my opinion like Bogie said on the seconary side I always adjust the secondary side to allow the right amount of air in relation to keeping the front in the correct range needed for everything to function like it needs to be.
I also use a spacer for clearance issues especially using a double pumper with an edelbrock intake and the vacuum fitting hitting the fuel bowl in the rear. I With the intake you have I recommend going to a 4 hole spacer vs an open spacer as the open spacer will make a weaker signal on idle and also while cruising and cause all sorts of tuning headaches that you will compensate for and on a street engine that is used for mostly just driving around, the four hole spacer has always worked the best for me as it enhances the signal to the carb in most of my experiences with using a dual plane intake. Now the thing I don't like the air gap intake the most is the cut down divider.
I have had an issue some time back within the last two years but my brain is foggy on which build it was, and I was using the air gap intake and I had no problems with the air gap design of the lower runner and the air flow going through it vs the non air gap design intake but did have a lot of problems because of the cut down plenum and it causing the signal to not be as effective and cause fuel distribution issues with certain camshafts and making a worse situation and a lesser tune vs a regular dual plane intake where both sides are separated from each other.
The sharing of the signals getting shared with each side through the firing order and then being pulled back and forth and getting cut around the cut plenum in my opinion ( I can't describe it good enough in words) makes for an unstable signal at certain times on the bottom end and mid range and even using a four hole spacer still does no help in that instance. I swapped out to an Edelbrock rpm intake and even though I still had my issues on said build with the cam I had, just the taking out the air gap cut plenum out of the equation made a big change for the better in trying to get my problem figured out which I was unfortunately unable to get fixed because of another issue that has me needing to take my engine out and checked out by a machine shop for machining error somewhere.
The edelbrock rpm air gap intake with cut plenum and a four hole spacer was worse then using an Edelbrock victor jr single plane intake and a four hole spacer combination on a previous build that was able to pull each style of intake off without effecting the performance and overall tune. In my opinion the cut down plenum is not worth the messing up the bottom end just to get a 3 horse power difference on the top end and maybe a 100 rpm difference in the overall curve of power being made. To me the edelbrock rpm air gap would be better with the plenum left alone and perform better that way.
To me in most cases the dual plane intake is better suited to keep the sides separate from each other in all situations and if you have to use an open spacer to get what you want overall on the upper end of things then going to a single plane intake is better off then using a modified dual plane intake and you will get better results overall but there is always take and give when going anything more then stock type builds.
If that was my carb I would be replacing the front butterflies with new plates with no holes in them and re adjust and tune things from there. To me those holes being there are just making things more complicated then they need to be with that cam. I ran the comp cam big mother thumper cam which has a massive 243/[email protected]
duration and only made 6 inches of vacuum in park and I did not have to drill holes in my carbs I used and one was a two corner idle and the other a four corner idle and I had my transfer slots on the front correct and the secondary side adjusted as needed and had a good idle required for what it was and no problems with fuel fouled and black plugs etc except the poor manors that comes with cams like those for just a daily driver type deal.