This is my opinion and my opinion only.
If your engine builder says that using stock log cast iron manifolds isn't going to hurt performance on a tunnel rammed big block with a cam in an A-Body, it's time to find another engine builder.
You will lose at least 50 or more horsepower alone using them with your set up.
Now, if the engine was dead stock, the hit wouldn't be so bad.
If I were you, I would look on BigBlockDart.com and do some reading, there is a lot of info on there about using stock manifolds versus headers (both cheap and expensive) and the fitment issues with all of them.
My brother back in the early '80s bought a '71 Duster with a mild 440 in it that had the exact manifolds that you pictured on it.
We swapped out them for some Hedman B-Body headers, and the difference was astonishing.
Sure, they were a pain to install, but the difference was night and day.