I run a set of Ed heads on a 496 with fast rate cam and 1.5 rockers (243@.050, .55" lift). With the 1.6's yours would be similar. I think I am near the limit for a standard single spring. You probably spin your engine faster... 1.6's may get you into a marginal valve spring/stability situation.

I am planning to upgrade springs on mine a bit as I am running it on the road track and pushing the RPM more.