I have a 79 chevy k10, and it runs fine, but when I go to shut it off it wants to keep running. It will shudder for a couple seconds then shut off kind of rough, and will puff a little smoke out of the air cleaner assembly. No idea what it could be, and it didn't use to do that.
Is the idle set too high? I have a '71 Ambassador that will do that if I don't get the idle just right. (A little tough to find the sweet spot since it has no provision to kick up the idle when the AC compressor engages, so it's more of a problem in the summer when I crank up the idle a bit.) Also check ignition timing and idle mixture. Carbon buildup can also cause this problem.
Some 1970s-vintage engines have an idle drop solenoid that's supposed to close the carb's throttle plate when the engine is shut off. If you have that and it's not working you'll get that kind of dieseling effect.
