My old Trans Am with the Pontiac 400 had a 750 cfm Quadrajet on it, 455 engines had 800 cfm units. My 308 Holden has an 800 cfm unit on it. The Pontiac 455 SD had an 800 cfm unit on it. Not one of these engines "needs" 800 cfm nor do any of them use the full 800 cfm on performance street trim.
I have posted the formula in a couple of threads on how to work out how much cfm is required. The formula is generic and is mostly used for a 1.5" pressure drop (4 barrel carbs) and 3" (1 & 2 barrel carbs and throttle bodies). IMPCO uses 2" (for the air valve). The formula works quite well if the VE used is honest.
So lets do your engine. CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456. 454 x 4500 x 0.85 (I'm being generous with VE) ÷ 3456 = 502 cfm. Now what will happen, regardless of the induction system, is once you reach the maximum flow of the chosen unit the pressure drop will increase. So say you have a 500 cfm Holley it should theoretically max out at 1.5" HG pressure drop once you go passed 500 cfm @1.5" HG the pressure drop will increase to 2" HG and so on. The same thing will happen with your IMPCO 300 and a 425 as well. The pressure drop increases because the induction system has reached peak flow and even though the engine is still increasing in rpm it has to work harder to ingest the required amount of air.
Early VW boxer engines suffered with this badly to the extent they were speed limited by the induction system as the engines were not powerful enough increase rpm at higher pressure drops.
EDIT: The cam isn't all that big for an engine that size. Usually the larger the engine the larger the cam it can support and still get "decent" vacuum. Notice
in this thread the 204@50 cam was consider an RV cam for a 4.9 litre engine but you have a 7.4 litre and the 214@50 is considered an RV cam. If you went to a 194@50 thou duration cam you'd probably get approximately 3 or so inches of vacuum more which would give you more low down torque and better low down throttle response. Using a "stock duration and lift" cam with higher ratio rockers would give you more lift without a change in duration which would give you a wider rpm range. Pontiac did a similar thing when they went from the Ram Air II to Ram Air IV engines, they went from 1.5 to 1.65 ratio rockers and gained 10% lift without increasing the duration or hurting vacuum.