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

Does NOS still offer the under-manifold system?
I don't know if they still offer it, but I was gifted a brand new 440 Sixpack manifold, because someone completely failed to manage an under-manifold hidden system.
A few welds, a few plugs, it will work fine n/a.
Hard to hide an underneath system on a bbm.