Are you sure that its the valve springs that are holding you back? Another likely culprit is the points; rarely do the stock distributor/points setups go past 5000 RPM before limiting engine speed. If you are running a stock distributor (meaning points, not electronic), you might want to invest in a set of high tension points.
If it is indeed the valvesprings, a good set will allow the engine to rev to 6000 RPM. The cam is capable, although you will probably hit your power peak around 5000 RPM, so going all the way to 6000 may not buy you anything.