Well, here's the lineup for the '79 motors......
L39 305 145 hp
Z28 350 175 hp
Corvette 350 225 hp

Since you have little more than a stock rebuild with the exception of the cam, I'd say you might be adding about 30 hp. The Holley manifold is little better than stock in my opinion, since it is a low-riser design. Of course, it'll be lighter than a cast iron manifold.

Hold the 305 heads for your next 267 buildup.