Extremely rough idle.

[Custom/20]

Hi everyone, I've been trying to figure out what is wrong with my 69 c/20. I've got the stock 350 that was rebuilt and 3 days ago it was running fine and all of a sudden it started running like crap at idle. I had it idling at 750-800 and its now idling extremely rough at 500-550. It smooths out at about 1000rpm and up but you can hear a contant faint back fire out the exhaust. I've replaced the cap and rotor, coil, wires, and plugs and nothing has changed. I got a timing light and its at 6 degrees at around 800rpm. I did some math and if the timing was set at 12-13 then if I'm correct and if it jumped a tooth it would be right around 6. I just would like to get some opinions before I go diving into it to put on a new chain.