Their intended scope is better then stock, but not all out performance.
What you describe is typical of them.
With 427 cubic inches, you can't be hurt with a single plane at all.
The main differences you will see is a little rougher idle and maybe a little flatter to 2500 or so but you will have top end to over 7000 rpm usually unless you don't have enough cam.
If you have a vacuum gauge hooked up you will probably see less idle vacuum but not to worry. 12 inches is enough. You can even get by with 10.
With a manifold like you have, you will probably not see less then 2 at WOT. Maybe not even lower then 3-4. With the single plane, you will.
A reading like that to me indicates there is restriction in the manifold at WOT. Again, typical of an Edelbrock.
Some have been able to port them for better results but that really depends on the manifold design.
Personally I have been playing with Clevelands since 1972. I have yet to find a dual plane FOR A CLEVELAND that will perform where it counts LIKE a single plane.
These are just my experiences and others may have different views and results, and as always, "your mileage may vary".
If I were you I'd ask Dan Jones if he has any specific manifold recommendations for you. He has dynoed quite a few on his 408. He could save you a lot of time and experimentation and put you right in the ballpark where you need to be.
The manifold you want may not be a current production one. I'm thinking it might be one of the discontinued Ford Motorsport/Racing units. There are a few of them. Dan has tested a few.
Your cubic inches will have different requirements then a 351 will.