You need special springs for a 303? Stock should work fine (unless they are bad). I had to get custom springs for my 347. But I bet your engine doesn't turn 9,500 RPM.
Not sure if this is the right answer but I installed a Comp spring kit in my old LIghtning GT 40 heads. It had the springs and different valve locks to make the spring installed height as intended on intake and exh, as the valve lock grooves are at different heights due to the exh rotators. Should be still available. I had a 484 lift cam, no issues using stock rockers to 6200 rpm.