Bad valve seals let oil in around the valve stem along with the fuel mixture. The oil is coming from under the valve covers which is splashed around by the rocker arms, springs, and push rods. During idle where the vacuum is highest, more oil is pulled in with the fuel which causes the smoke. When you take off from a long idle, engines really smoke heavy till the oil is burned off. You might not notice as much smoke while driving due to the vacuum variations and the amount of air going in. Also, long decelerations like down hills, or coming to a light pull high vacuums, which pulls more oil. With the carburetor throttle shut, the engine is trying to pull what ever air in it can get, this includes air inside the engine.
Its been my experience that bad rings cause more blow by then smoke. If yours is blowing oil out the valve covers, seals, and vents, then the rings are not seated or the cylinders are not round. They tend to use oil in this condition, but the true sign is lots of blow by.
You should check the cylinder walls and ring gap at the top of the bores, thats where all the wear is. The most pressure from combustion is right at the top so the rings and cylinders wear the most, by the time the piston reaches the bottom, most of the force is gone and the exhaust valve has already opened relieving the pressure.
If you look at the exhaust ports & manifolds and notice a black sticky goo, thats burnt oil. I have seen exhaust valve stems built up with so much crude they look like a solid slug the same diameter as the valve head!
You need to look between the valve spring coils and see if you can see a seal. If not, pull one spring on one intake and one exhaust valve to verify they are or are not there. If by chance it has white teflon seals, throw them away and go get a high temp Viton seal.
As mentioned above, most heads will need the top of the valve guide boss cut for a seal. It's a very common operation and can be done at home with a decent drill or drill press. Any shop should be able to do it also. I can't believe it wasn't already done.
