340 Running super weird

If you can get the timing light on it and give us some specifics, we may can help. Where the initial (idle) timing is for example. Also when (at what RPM) the mechanical advance starts to advance would be helpful information. Also, details about the engine build. Compression ratio (known MEASURED, not a guess) camshaft specs, converter stall if automatic and rear gear ratio. If you do not know the compression ratio, some compression test results can get us on the right track.