440 duster smokes sometimes???

i just bought a duster with a 440 engine. it runs better now than when i got it but i drove it threw the parkway here in NC and as i was going up the mountain i could smell smoke and could see the car smoking behind me. i pulled off and opened the hood and it was smoking on the driver side near the exhaust manifold. i could not tell exactly were the oil is coming from. there was some oil on top of the valve cover. i changed the valve cover gasket. i have not drove it up in the mountains like that again but i did get the car running right or better and was able to open the 4 barrel up for the first time and when i did i smelled smoke and it left a rather long and large smoke trail behind me but cleared up when i got out of the throttle. what could be causing this? im new to dusters and 440s. thanks