whats wrong with your friends 421B? That engine is more powerful than a WS7II. All of those engines are top end obviously and the 421B can run with them no problem if tuned properly. The V-spec is nice because it has great low end as well - but i personally wouldn't run one due to the fact that too many people have had problems with it. Also, the Crono RS7 is a finicky tuner and apparently struggles to hold a solid tune, despite its also considered one of the fastest engines you can put in a buggy.

Its really up to your friend and what he wants. Obviously something very fast and geared for top end power, but depending on the engine, reliability and efficiency made need to be sacraficed.