1969 Olds shuts off at stop

some older vehicles, i adjust the idle in drive so it won't stall when you stop. You could put a carb kit in it and adjust the idle rpms to spec, see if helps? The idle specs used to come with carb kit. the only problem I see with adjusting idle in drive, when you put it in park or neutral, the engine races.

Look and see if there is a solenoid that controls the idle speed.