i own a repair shop,and i think in that year they were gt 350,s i don't remember one being a 500,,because i owned a 350 before,,but if built right they,d run anywhere from a 125 - 150,,mine ran 145 wound to the gills ,still got the speeding ticket to prove it also,it was given to me on the night of what we all called the big race there was over 60 races ran in one single night,and i won a few of them.and lost a few of them also,but the last ten races were title races i won a 1971 dodge challenger ,in one of the races,and still have it to this day,,good luck,i hope this help,s.