I am pretty sure that the hottest Cleveland heads flow in the 330 @ .600 lift range BUT I'm not sure that a 351 can use that. Flow isn't everything.
I think anything that flows 285 or better at 500 to 600 is pretty decent. Those were numbers that only serious race prepared engines were reaching 20 years ago.
I think you need to look at some engine dyno tests and see what the results suggest with different intake manifolds, cylinder heads and camshaft combinations.
What big cube engines do is put a mountain of torque right down off of idle.
I don't see how you could go with more cam. Induction would be your biggest choices at the moment.
As far as saying 2x4 Holley system doesn't work as well as a single Holley, that depends on your interpretation.
The advantage of a 2x4 Holley is that you are driving on a smaller set of primary throttles, in the primary carb, up to 3,000 rpm.
There the primaries on the secondary carb begin to open.
If you are using say 1850's, those are 600 cfm carbs. The primaries on them are smaller then the primaries on an 830cfm Holley and the better throttle response shows that as well as better overall milage.
The secondaries are vacuum operated and will open in unison to the vacuum demands the engine creates.
The 1850's are also a street, original equipment carb from Ford. I have run the pair with as small as 59 main jets and the plug colors are perfect with them. Try to run those jest with your 830, and you will put holes in your pistons.
The old numbers from Ford indicate that 2x4 carbs will make 50 hp more then a single carb.
I didn't understand all of that until I tried the system. Now I understand what they were talking about and the system is much crisper, has a tremendous top end, and gives about 4 mpg better around town then one big Holley.
Also keep in mind that two 4's was the system of choice in Trans-Am racing until the rules outlawed them. Not just one big monster single carb.Do you think there is a reason for that maybe?
In all fairness to accuracy on the subject though, what really makes the Ford system work is the design of the intake manifold. It just happens that the only combination they could make fit WORKS, and it makes the engineers look like geniuses.
Chevy's 2x4 don't work any near as well as the Ford design. Has to do with port layout, etc.
Also Bill Price really did a wild and crazy thing. He copied the manifold design of the 289-302 and extrapolated it up to a 351?
No they work and really quite well. Oh and not that I need to put icing on the cake but ever hear of the Ford GT40 MarkII and Mark IV? Those are the Ford 427's. Guess what induction system they ran in competition? You'll never guess?
Hey Doug, what is this a conspiracy? Geez-us! 
I understand Mr. Ferrari didn't like them either? Said they were irrigation engines? Imagine all those linguine wagons painted tomato sauce red beaten by a bunch of pion Americans with the irrigation engines they took off of the corn farm? Is there no God in Heaven?
I have Weber 48ida's also and they both seem to make about the same power on top, but with the 2x4's you can virtually pop the wheels off the ground in the front, the Webers won't.
The Webers have a better throttle response in the 5,000rpm range (around 100) but not by much. Webers have the reputation for having better throttle response in very high speed cornering and I think is well deserved.
In real driving both the 2x4 and the Webers will blow the single Holley away. No contest at all. On a dyno there may be different results, but I don't know how to drive a dyno on the street or even the race track. Do you?
It all depends on what kind of driving you do or how you drive the car.
The Price manifold is $450. Plus the 427 Ford throttle linkage at around $100, then the two Holleys. If you use 1850's you can find them "factory rebuilt" for about $150 each.
I don't think that's so bad? A 351c Weber manifold these days is $950 new, carbs $650 each. A Weber setup will cost you over $4,000 depending on the particulars.
BUT, and this is a big but, you already have a dual plane manifold and it shows signs of restriction. The point is you can get some more top end and basically keep the bottom end you have by going to a single plane manifold.
None of these are mistakes, they are choices that you have. Nothing has zero drawback. Some of the drawbacks you won't know of until you have a system running and drive the car.
If you don't like the drawbacks or limitations, then you go to plan "B"...whatever that is?
Try a single plane manifold first. Maybe it will be just what the doctor ordered?