A 2x4 Holley set up is similar to a Holley single "double pumper" set up but with vacuum "afterburners.
The FORD 2x4 linkage makes the primaries on the carbs open progressively just like a double pumper does.
The secondaries start to open around 3,000 rpm but open twice as fast as the primaries do in order for them to catch up and be synchronized at wide open throttle.
The vacuum secondaries do not really start to open until a little over 75% or so of the primary throttles.
So even on a 289, IF you can run a 4778, aka a 600dp, then you can run two 1850's.
Now in fact, those two 4's are actually cleaner then the single 600 dp by a long shot. They run 54 main jets and 119 secondary plates where as the double pumper starts at 69's with 71's in the secondary and will smart your eyes at idle because of the "competition" heavy idle. Set up that way to take advantage of the scavaging of competition headers.
The AFB dual four set ups are what you want to stay away from. Those have different linkage set ups and you tend to be running on the two of them all the time instead of the Ford set up of a primary and secondary carb.
If anything, you will find that the 465 Holleys are really too small for the set up.
Additionally, you will most likely wind up with a "Blue Thunder" version of the Ford "high rise" manifold. That manifold has the ports closed down near the intersection to the cylinder heads.
If you run it that way you will find that it is restrictive and unimpressive performance wise. You really need to port match the manifold to the head gaskets. Then you will see significant differences.
The neatest thing that I find is when the secondaries open you will hear this "whomp" sound like the sails opening on a racing boat does. The response is anything but a bog. The thing pulls big time.
The C6OA T/A manifold is a little bit of a different animal all together. On a dyno you MAY show that you are loosing HP off of idle but gaining around another 20hp over the ported "high riser". (Randy calls that one the "turd").
However take seriously the SA advertising that the "high riser" is good for another 55hp over the original single 4. I think that is conservative these days because of not necessarily being limited to the original camshaft profile.
The original single 4 is only good for around 12mpg cruising. If you beat on it, forget about that. Probably around 8mpg.
If you want to play "trans am racer", you will get what you will expect mpg and power wise, but that's up to you.
The 2x4 Holley set up is very streetable although I'm not sure some drivers are and there are some that have money to do this but are not capable of handling the car at all afterwards because of the additional power.
They do not pull like a Weber set up does. The power comes on in sudden stages. The Webers are just smooth all the way through.
Your biggest problem, if you want to call it that, is whether to run the carbs facing forward or backwards like the FE's do. Backwards has it's advantages and disadvantages but was proven to be a streetable linkage.
Forward was intended to be used on the T/A race cars and my thought is that it is there to simplify the linkage and save wasted time needless spent on a racecar.
Your choice. There is no right and wrong. It will just move the front carb 1/2" closer to the distributor cap and almost touch it. You feel confident that you will never develop a crack in that cap and short to the carb right?