OK then.
Sizing the carburetor will depend upon the normal variables with the actual displacement and the cam selection being first in line followed by the expected engine red line value.
If the choice is a 4 barrel, I prefer to run Holley carbs with Vacuum secondaries. The metering plate that sets between the rear bowl and the main body can be replaced with the same part that sits in the front. Holley has a kit for this as the fuel tube connecting the two bowls requires replacement as well.
By replacing the metering block, you gain the ability to tune the secondaries. The spring in the vacuum dashpot can be replaced to control when and how fast the secondaries open. This way the jets and the power valve for the rear portion can be changed out as needed.
In a Holley for a 4.8L without my knowing the specifics, but assuming a mild cam with a 5250rpm red line and the 4.8L displacement, I should think that a 470cfm 4bbl model series 4150 (p/n 0-90470) would fit the bill.
It is easy to make the mistake of installing a too large carb. Resist the inclination...
As an example, in the 2bbl application, what is available (if memory serves, getting Senile...) there are 350cfm and 500 cfm versions. The 350 is too small to be worth purchasing and the 500 is, IMO, too large and will create tuning issues.
To be fair here, I am not conversant with Edelbrock products but have to suspect that they make something similar. I have favoured Holley as it was the factory carb for more than one US manufacturer over the years.
Hope this helps...
BTW, you will need to acquire a vacuum gauge so as to be able to set your idle mixture. The "high speed" mixture will have to be tuned by reading the plugs. Either that or, install an AFR gauge and do it electronically. Old School is less expensive but the New School is more accurate...
If you chose the twin carb setup, Holley has the 5200 series which is a licensed copy of the Weber 32/36 series. In both cases, the carb is a progressive two barrel with water heated chokes. Keep in mind that you are creating a "pair" of 2.4 liter engines as far as the carburetion is concerned.
There are advantages to multiple carbs. The primary is fuel flow to cylinders 1 and 6. With a centrally mounted carb, these cylinders tend to run a tad lean as the fuel wants to drop out on the way. This is especially true if the manifold contains sharp bends in it (i.e the factory log manifold). With two carbs, this effect is reduced. The port locations do not readily permit the use of three two barrel carbs which would be best for performance as it allows tuning the individual cylinder.
I looked at the Offy Intake and it is a dual port, not a dual carb model. IMO, this is what used to be called a "dual plane" intake manifold where the primary runners are separate from the secondary runners. Which is a very good thing indeed.
Why is that important? It has to do with the velocity of the charge. It is important to keep the velocity as high as is possible for this reason:
Any working fluid in any conduit tends to form a vortex as it runs through the conduit. The higher the velocity, the tighter the vortex becomes and the more likely the fuel will remain suspended int he middle of the vortex.
I would suggest that the OP acquire some reference manuals and read up on it. it is tricky to get right. Personally, I would convert to to fuel injection and call it a day.
I was not able to locate any sort of multiple carb manifold. No demand any longer I suppose...
