ive heard 430 or so max for the stock ones off a truck, and the new summit catalog says .475 now, but said .450 or something last month... the scoggin dickey catalog says about the same (.450, but i dont remember right and dont want to get it out...)
BUT the new GM performance parts catalog says the gm vortec performance heads, part numer ending in 060 i think, can take .475 stock... so either way, .480 wont work unless you get the upgraded heads from scoggin dickey or gmpp makes an upgraded valvespring version too...
if you have already bought some vortec heads, you can buy a tool that will machine the 3 things you need machined at the same time for the bigger diameter valvesprings from crane cams i think... (i believe comp also makes a tool that will do some of the machine work)
or you could take them to a machine shop and pay a little more $$$
good choice on vortecs tho, theyre good heads, and for 500$ theyre a bargain cause the flow numbers are better than some aftermarket heads that cost 400$ more.... i just put a set on the 400 im puttin in my 83... (goodbye 305