If the dwell changes with rpm, it indicates that the distributer shaft bushings are worn, a small amount of change( 1 degree) is normal. Timing should change with rpm but dwell should remain steady. My '57 5700 with 283, stock carb and distributor has a spec of 4 degrees BTDC static timing and has no provision for vacuum advance.
1957 Chevrolet 5700 LCF 283 SM420 2 speed rear, 1955 IH 300U T/A, 1978 Corvette 350 auto, 1978 Yamaha DT175, 1999 Harley Davidson Softail Fat Boy
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