Actually, slight head work and decent carbs do not require double-valve springs. The stock springs are perfectly fine for 6,000 RPM and some runs them to 6400 RPM. They start floating about here, so you'd need upgraded springs to rev higher

Some guys refuse to rev their engine over 5,000 RPM, which is why I asked. If you like to rev and are willing it gives you much more choice and potentially more power