these original blocks can hold more power if the engine is "boosted", is that true
Yes, the cylinder pressure "spike" (BMEP) is lower and more widely distributed than equivalent HP NA.

Many successful turbo race cars don't use fabricated tube headers, but simply use modified stock cast-iron manifolds upside down, reversed front to rear etc. Google around for 1968-70 B/RB manifold choices on turbos including truck, C body etc.