Miles per gallon

Depending on the condition of the engine, a 42 year old car, if it has the original engine, you will get about 12-15 city and 22-25 highway. That's optimistic. For the record I did not guess, I looked it up. Mow may miles? If it has say 200,000, which would be not unreasonable for that old of a car (a genuine classic, keep it) it probably needs piston rings and you will get better, but that is so expensive I would not do that. Replace engine. But all moot if it runs well!!

most guys pull the 307 and replace it with a 350. if its in good running condition it will get pretty good mileage. but not much in power.