The Viton tipped needle & seats are the typical standard needle & seat the common Holley carbs come with, and come in the Holley renew kits, so yes they are fine for street use. Holley recommends the steel inlet needle & seats for “exotic” fuels, like alcohol, etc., but I have used them with race gas without an issue.
You didn’t specify exactly which 600 cfm you have, but for the common List 1850 the original needle & seats are Holley part # 6-506, and have a .097” orifice size. There are numerous versions of a 600 cfm Holley, so a good idea to double check your model.
The genuine Holley needles & seats are not cheap if bought individually, for just a little more $ you can buy the whole rebuild kit, than what a pair will cost, just FYI.
Our member Drew at Air-Fuel-Spark is currently home and in the shop rebuilding carbs, he is selling his own custom rebuild kits & parts, you might want to check with him too.
