Edelbrock carb running rich, sbc?

First off I have a 79 chevy truck with a bone stock 305, carb and performer intake is all. When I bought a new 600 cfm, some people said It would be fine and others didnt, I was thinkin more like a 500 cfm. My truck now, runs good and all, but when you rev it twice kind of slow it will puff black smoke out, and coming to a stop, (its a 4 speed manual), without downshifting, and hittin the brakes hard it will idle way down and almost die. So im thinkin too much carb for a 305. any ideas?