I'm going to recomend getting in touch with board member Jeff Patterson. He deals Engle cams, has a wealth of experience going fast, and is willing to help you through a choice.

While I'm sure its a decent choice, the specs given here on the RB ST-21 seems like it gives away some lift ramp to some newer grinds. The Cam Jeff got me for my 451 gives quite a bit more lift with slightly less duration (???).

Good luck with whatever you end up with, sounds like a great build. I also think your under estimating what that combo will do.