4 Barrel on a stock slant

By that formula every car is over carbed.

A 650 holley front barrels is the same as a 390 two barrel and a 750 front barrels is the same as 500 two barrel.

I don't think anyone would argue about putting a 390 2bbl on a /6, since the vacuum secondaries only open as needed can't seeing it cause too much problems.
A 4bbl with smaller primaries would be better. The gains would probably be small over a stock 2 bbl. The only way is check the restrictiveness by the vacuum level at full throttle run and go from there.

Don't forget a just because you throw on a 1000 cfm carb on a engine that only needs 500 cfm that is not gonna give twice the fuel and air to the engine over a 500 cfm carb.

Carb size is more about restriction then it's actual flow numbers since the rating is based on vacuum levels 1.5 for 4bbl and 3 for 2 bbl. So if your engine produces a different vacuum level at full throttle then you carb will flow more or less then rated.

So if your engine see a vacuum levels of 1.5 with the 500 cfm carb and then you throw on a 1000 cfm carb the vacuum will drop so the actual cfm of the 1000 cfm carb will be about 500 cfm too. And on the dyno would probably gain hp only because of less restriction but wouldn't work cause of drive ability, idle and part throttle.