If you don't have the owner's manual, a common recommendation for a 305 engine in that era is 10W-30 conventional oil. However, 10W-40 is also acceptable, especially in hotter climates or with higher mileage. Always check your owner's manual for the most accurate information.
Before changing your oil, be sure to check the owner's manual for the correct amount of oil required for your specific engine. Using too much or too little oil can damage your engine.