Hello rated x,
I started with an 850 Speed Demon with vacuum secondaries on my 521. It stumbled, stuttered and bogged so much that it was extremely frustrating. That particular carb is NOT recommended. I went to Dahlonega and tried to get help. I let some "experts" fiddle with it. No joy.
Finally, I switched to the MPFI from Holley. It works well - after some sorting, but not near as much as was tried on the Demon.
If you're determined to stick with a carb, you might want to try a 900 or 950 Holley with mechanical secondaries. Edelbrock makes one too.
Yet another $0.02
Tom