1977 Chevy 350 Smokes till Warms up, then quits?

[LBJ]

On my 1977 Chevy Bonanza with 350 and automatic tranny, engine miles estimated to be around 75K to 90K miles with nothing major done (disconnected the speedo in 1980 until 2007 because it was making a big racket) it will smoke out the tail pipes until it reaches the operating temp setting on the gauge, which corresponds to the thermostat opening. If I start up and just start driving, it is usually warmed up in a few minutes and quits smoking. But, even sitting in the drive and letting it run to charge the battery after sitting for weeks, it will smoke and then magically quit. And, more, if I drive it regularly every day, as opposed to sitting for weeks between drives, it won't smoke even when dead cold. Now, what could be the cause of this? If it was rings, it would never quit smoking. Valves? Maybe valve guides are worn, but that's about all I can think of. Any ideas? It's not a problem worth tearing down the engine or pulling the heads, but I'd like to know why the heck it does this. Ha. Thanks for any tips. LBJ