Does it have a real HEI, or the Pertronlx junk? The GM HEI distributor housing does not move. A Pertronix-equipped original distributor housing will move just like one with points when the vacuum advance operates. Yes, an inop vacuum advance on either type of distributor will drop the gas mileage by a noticeable amount. It's easy to test either one- - - -use a long piece of vacuum hose attached to the unit and pretend it's a straw to a thick milk shake. On the GM HEI you'll need to remove the distributor cap to see the advance canister linkage moving the pickup coil.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!