GM had a special tool to loosen the slotted nut that's down in the recess in the outboard insulator. A work around is grinding a gap into the center of a stubby straight bladed screwdriver to clear the threaded stud, or filing a piece of 1/4" steel fuel line to leave a couple of projections to engage the slotted nut. The vintage OEM tools are usually available on Ebay.
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!