Handful of guys here run 22s with XR drives and Imco 2" shorties. Pretty proven setup and I expect you'll get lots of good help. I agree that I'd expect you to get over 80, though if it's all only been tested in the hottest and most humid part of the summer, perhaps not. In anticipation of more dialogue with those guys, couple questions come to mind.
Did you you actually get dyno testing that showed 600hp or was it just built with that as a goal?
I presume you have 1.5 gears? I ran your numbers and it sounds like you're slipping a LOT. More than I'd think you want. Here's that result if it helps with the prop planning: 18% slip at 5400, 1.5 gears, 28" pitch and 78mph
Regards,
Mike
"I don't have time to get into it, but he went through a lot." -Pulp Fiction