What octane gasoline should be put in a car with a 305 engine?

If you haven't raised the compression, the best fuel would be regular. There is absolutely no need for premium fuel unless the engine has been modified and the compression is above 11-1. Companies will try to convince you otherwise, but don't waste your money. The reality is, your vehicle will run far better and with much less need for repair if you only purchase fresh fuel. Most stations pump far more regular than any other grade of fuel. That makes the regular fresher and assures that it will give you less problem.