Four-barrels like the 390 are tested at 1.5 inches pressure drop, and if the 500 two barrel is tested at the same pressure as the four barrel, it flows at 353 cfm - smaller holes give higher air speed which encourages better fuel atomisation.
Four barrel carbs don't seem to give good results either, a 390 Holley is just too big.
The 500 Holley has a flow rate of 500 cfm at three inches of water pressure drop.
For the Capri, they homologated the Weber 40 DF15, quite good for racing but no good on the road, as they don't have a power value and give a very rich partial throttle running - washing your bores out quickly.