Lightweight flywheel options and selection criteria.

For your application I would use the 20 lb steel.
I did run a 12 lb aluminum wheel behind a 370hp LT1 in my 57 chevy, and I LOVED it! It revved so quick it was amazing, and I was still running the plastic 8.2 dropout rear end, so I did NOT need the heavy flywheel transferring a lot of torque. And the 4.56 gear helped too. A scattershield was ABSOLUTELY MANDATORY.
The light flywheel helps with acceleration as well, especially in the lower gears. Much more so than even lightweight engine internals.
Imo, a heavy wheel is only necessary for a car with almost no gear (2.76/2.91, etc)
Edit: whatever you get, get an SFI approved version. (Have you ever seen a flywheel/clutch explosion? I have. UGLY!)
Edit2: I heartily endorse a scattershield too. Most racing sanctioning bodies require one, or should.