360 head porting for big power (500HP ?)

Some simple questions for anyone 'in the know'?
Valve choice for J heads?
Tulip or Stainless flatties...
I got Milodon, Ferrea, cheapo chinese and good old 'factory Tulips' as its spring lol.

I can't go to 'all out' portwork, LIKE SKREWS, but how far can you go, around the original guide boss???

Thanks from the Limey