I'm with StorminMatt, my last roll of Kodachrome 64 was on a gray cloudly day, and the slides were just dreary, worse than reality. Nice film with bright light, but you need that light. Since then the only Kodachrome I shot was 200 and 25, and I've used up my supply.
Velvia 50 as a substitute? No way. That's a special-effects film in my book. I liked E100GX (discontinued), I presume E100G with an 81A filter is pretty similar.
Given a good scan, one can get Ektar 100 to look more than a little like Kodachrome.