Street Demon tuning experience

My suggestion is forget about the WBO2 for now. Hard to trust them, especially at low rpms. Sometimes they are close and sometime not.

Focus on getting a good idle. One that results in minimum drop in rpm as you engage 1st (or drive).
With 20" Hg at idle, assuming that's like 600-700 rpm, its may have close to stock power at idle. Maybe better than stock in the smog era.
That is a lot more vacuum than most hot rods, so will result in stronger pull on the low speed circuits.
But if the throttle position is ballpark correct, then the idle mix screws should allow you to adjust for a clean and strong idle mix.
If the screws don't provide a level of control (for example screwed until seated and the engine is still running) then the throttle position is too far off.
You can give it a little more or less timing to adjust the rpm if you have to. I'd guess 12-16* BTDC around 650-700 rpm is where it will end up.