I built a 20r 22r hybrid years ago.........didn't work out like I expected, and norbie's right 21r heads aren't as good as 20r heads. Forked out fair amount of cash, hi tops, cam, side draught etc.....just never went very well.
Still always felt like a ute moter.
I ended up ditching it and swapping for an 18rg, which was a fair bit better, but not a "bolt-in" fit.