SLANT 6 timing issues , In need of a Mechanic , los Angeles Area!

The smoking sounds more like it had a carburetor problem. You shouldn't have paid to have the engine rebuilt without first checking the compression, which may have been fine. Indeed, you can check by turning the engine over by pulling on the fan belt. But water under the bridge now. My first car was a 1969 Dart, bought in 1976. I was a dumb college kid then and didn't have relatives that knew cars, and there was no internet then. Don't believe stories that men knew cars in those days. When I was young, most didn't even change or add oil.

Like you, I had the engine rebuilt after a perhaps minor problem (oil pressure lamp on, probably just gunk on the screen since went away for 6 months). The car never idled smooth and often died with a little accelerator, and much worse on hot days. No mechanic could solve it. I had a "valve job", then rebuilt engine, changed intake, finally whole new long block, and it still idled rough. Finally tried a 4th Holley 1920 carburetor and it purr'ed like a kitten and no longer stumbled when leaving a red light. Many rebuilt 1920's are bad, having clogged internal metering blocks that aren't rebuilt. Find a guy who rebuilds carburetors and is recommended, or change to the later Holley 1945, and report back. Or, read-up on the Super-Six 2 bbl intake/carb for more power, or the 4 bbl intakes. Since spending money anyway, you might want to upscale.