Probably the old FE 390. 600 pounds of power ;) The Chevys of that vintage were pretty beastly as well. Had a friend with a 68 stepside and an I6. Thing got like 18 mpg and refused to die. And it pulled like a mid sized V8.
When I worked in Tampa many years ago my boss had me drive all over town in his 70 Chevy pick up with the 6-gun. It was no Z-28 but that engine is virtually indestructible.