That engine was "stock" only about a month after I bought it, about 1972. Best mileage was with 1972 340 heads cam and valve train, 1971 340 intake and TQ, and 273 with TRW forged 10.5 pistons with windage tray. The 72 heads were not untouched stock either.
A efficient combination. If you had a o/d back then you would probably pick up another 5 mpg. Stock engines with a low rear gear or a regular gear and o/d make a difference.
