Rich;
HRL is right on if the rings are gone watching the oil level is cheaper than a rebuild. If the engine has a lot of blow by and uses oil then it is probably the rings.
I would also take a look at the valve guide seals. The early SBC had rubber umbrellas for valve guide seals and with time and heat they may have split or worn. If the engine has been running recently and you don't suspect stuck/worn rings then the light brown plugs give me the thought that it might be valve guides and/or seals. During deceleration the high vacuum present will suck oil down the guides if they are worn and/or the seals are flaky. The oil is burned and produces the smoke cloud but when the engine returns to a load the combustion process cleans up the plugs somewhat and the cloud goes away because there is less oil present during combustion with the lower vacuum levels.
Another thing to check is the PCV system. I think in 1958 it was still a road draft tube but if someone has retrofitted a PCV system, it may be sucking oil on deceleration somehow.
Either way, whether rings or guides, keep an eye on the oil level.
SimS
