Carb CFM.. Increase?

If your 408 was able to allow the atmosphere to fill the cylinders on every revolution,to 100% atmospheric pressure, it would draw in 408/2=204 cubic inches of air on every revolution, which is .118 cubic feet.
At 4000 rpm this is 472 cfm.
At 5000 it is 590cfm.
At 5500 it is 649cfm.
At 6000 it is 708cfm.
At 6400 it is 755 cfm.
Your 254* cam might torque peak at about 4000, and this is about where it might allow 100% filling. It might even allow more than 472 cfm; perhaps as much as 520 @110%VE.
But as the rpm goes up it will be more and more difficult, especially if the engine is forced to ingest hot underhood air. Or is pumping exhaust thru straws. By 6000 it might be down to 95% or less, and that 708 might be just 673cfm
What this means is that you could have dual 750cfm carbs on there, and the engine will still only pull 673@6000 thru them..... because that is all the cam and combo,is gonna allow.
Now if you restrictor your engine as you call it, with a 650, which is rated at 650 at a certain pressure differential, that 650 with different pressure differential is gonna flow a different cfm.

But I gotta re-iterate, on the street, with street tires in stock tubs,and no traction aider; with your 4.30s and 3500TC; you are gonna be spinning all the way thru first gear; so you might as well have a 2bbl on there. 60mph in second gear and not spinning,if that is possible, will be around 4900rpm which being 590cfm at 100%VE / 650 cfm at 110%VE. So,in your combo, on the street, the math seems to indicate that a 650 will be just fine.
Now if you take second gear to 6400/almost 80 mph, well then she is gonna want a 755 times whatever VE your engine can generate there, say 90%, and that would be ; 755x.90=680cfm.

Now; of course, we all know Mopars don't follow this logic, and they some how magically want carbs that are at least 20% bigger. So if that's true you better put an 816 on there, cuz if you don't, the engine will lay down after ~4000, and the guy on a moped in the next lane, is gonna surprise you. lol.
Bottom line is; give the 650 a chance. Whats it gonna cost you?