IMO a 390 is a teeny carb better suited for 2.3 liter four bangers.
Nobody on this board would recommend that carb for a 5 liter V8; why is it the proper size for a 5 liter I6?
Noooooooooo! Not again!
The 300 has a significantly lower redline than the 302. And the heads don't flow nearly as well. So while the engines may be of the same displacement, the "out of the box" appetite for air is significantly less. That having been said, it makes no sense to put anything bigger than a 390 on the 300 unless you've done the requisite head and valvetrain work
necessary for the higher airflows.
The Carter/Edelbrock AFB: Yes, it is a simple matter to change metering rods vs. the Holley Jet system. But the AFB is a MECHANICALLY actuated secondary; the secondary throttle blades are linked to the accelerator linkage, and start to kick in at 65% throttle whether you want them to or not. Airflow through the secondaries is controlled by a pair of "air valves" that are regulated by a counterweight. This is a very crude method, and generally doesn't work worth a damn. (Think of the counterweight system on an automatic choke. The old Series "D" AFBs omitted this problematic system altogether.) And a counterweight system is going to be totally useless on an offroad vehicle subjected to all kinds of off-attitude "input."
Another thing to consider is the mixture differences between the primary and secondary sides of a carburetor; the secondaries are jetted richer to avoid detonation problems at full power output. If you overcarb an engine, you are delaying the opening of the secondaries due to the increased air volume necessary to open them. This delays the point at which you introduce the richer mixture, creating the potential for all kinds of detonation problems; overcarbed engines are real buggers to tune and make run right.
The Holley carburetor addresses the need for secondary fuel enrichment through a complex system of vacuum actuated secondary jetting and power valves. While this increases the level of difficulty in tuning the carb, it does provide for a much more precise and efficient way of tailoring the fuel needs of an engine under a variety of different conditions.