Get a disc clutch - you won't regret it.
Drum clutches will work alright if you're just getting started or just wanting to make laps and have fun.
If you want to get serious, you'll want a disc clutch.
I prefer genuine Bully clutches for performance and value.
Engagement should be at (or near) peak torque for your engine.
Peak torque will vary based on your track's definition of "stock" predator and their rule on peak rpm to a degree.
If you do not know where peak torque is on your engine, then I'd suggest starting @ (2800-3000) and adjusting from there.
I could be wrong, but I believe straight out of the box (no governor turned up, etc) they make peak torque considerably lower than this (ie 2500.)