You could have a machinist make a 7/8 NPT adapter, or enlarge the hole to 1" NPT with a pipe tap. Tapping the hole out would be the simplest solution, as 1" to 1/2" NPT reducer bushings are a standard hardware store item. Of course, if you modify something GM quit making 70 years ago, all your kids will probably be born naked!
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!