Those holes go into the exhaust ports and were for the "Air Injection Reactor" tubing. Air was pressurized by a belt-driven pump and injected into the exhaust stream to assist the catalytic converter in cleaning up the exhaust gases. It was a routine thing to rethread the holes to 1/4" pipe thread and put brass pipe plugs into the ports when removing the A.I.R system.
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!