I finally figured out how to make scrolling credits in 59.94 HD look good when downconverted to NTSC. I new 120 pixels/second was the way to go for NTSC but I never even though to increase that rate for the HD. For center cut and anamorphic downconversions it's 270 pixels/second and for letterbox downconversions it's 360 pixels/second.
I'm sure this has been posted somewhere. I could never find it though. I just realized that the HD image is much larger than 480 so 120 pixels in HD does not equal 120 pixels in SD when downconverted.