My 73 Ventura is fouling plugs

I don't have a high performance Pontiac just yet but I have a 73' Ventura with a Pontiac 350. It was all original when I purchased it in March of 09. I replaced the original intake with a Edlebrock performer, the 2 bbl carburetor with a 4 bbl Edlebrock 650, installed headers, dual exhaust, new cap, rotor, coil, plugs and wires. I got everything installed had it timed and started it up drove it for 10-15 miles then parked it. Wanted to go for a drive 2 days later and it was hard to start and hesitation when accelerating. Checked the plugs and the plug closest to the firewall and the plug furthest from the firewall was fouled on the drivers side on the passengers side it was the 2 inside plugs. I replaced them re timed and again it drove fine for 2 days. I let it sit for a day wanted to go for a ride and it was hard to start again and the same hesitation, checked the plugs and they were fouled again, same plugs . Its like a sooty carbon build up and there was also some light black smoke coming out of the exhaust. I'm not really sure what is causing it and I would greatly appreciate any help so that I can get my car running again.  

Black smoke out of the exhaust is too much fuel.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Thanks I will try to lean it out and see if it helps.  

Thanks I will try to lean it out and see if it helps.  

Your welcome.  
professional hi-performance engine builder

Horsepower sells engines and torque wins races

Where are you setting the timing? Might want to advance it a bit if you have hesitation.Might want to get a hotter plug also. the 650 is a lot of carb for the stock 350.