If you are under 2,000ft elevation then its barely OK
18" to 22" is expected, but the scale drops by 1" per 1,000ft elevation
So in Denver, 17" would be the top end of the scale, 5,000ft 13-17" expected
In 1984 models, spark timing was set manually so may need adjusting
If engine has 250k or more miles, the running compression will be lower so lower vacuum
But 17" steady means engine is still running OK
Many other tests you can do with vacuum gauge that can tell you if its low for a reason read here:
http://www.gregsengine.com/using-a-vacuum-gauge.html
Also, grain of salt, vacuum gauges are not calibrated that often
You can test gauge on another engine to see what it says
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