Rams horn manifolds will not work in a Chevelle chassis. The outlets would terminate right into the front frame crossmember.
To be real honest, I don't think any of the small block manifolds flow all that well due to their size and shape. Some may be better than others but I don't have the info to recommend a specific set.
A set of headers, even shorty headers, would really help that cam make use of it's power band above 4500 RPMs.