[morethanstock66]

If you want good gas mileage, out of those three, the 250 is the best option. It is also a really good engine-strong, lots of torque, ect. As you say, you won't get awesome gas mileage in a 60s truck, but IMO 250 is the best option. You can build up I6s to put out power also, later on down the road- offenhauser, Clifford, ect. I had a 250/sm420 with 456s in my '67 for 5 years.. And I drove that set up in high school also lol.