Can I use B or C body manifolds fore better flow on my sleeper GTS?

Imo, wondering how much power “can be made” while retaining the manifolds is the wrong approach.
On a big cube high CR race combo(for FAST), they’ll support over 600hp.
But that isn’t relative to what most street guys are doing.

The question should be, can I make the power I want with my proposed build while retaining the ex manifolds?

If the expectations and the build plans are on the same page, you should be good.
On the other hand, If you’re trying to make 500hp+ on pump gas with a hyd cam and ex manifolds....... you’re probably going to be disappointed.