I ran a Comp Cams Magnum 292 in my A-bone 454 with oval port heads, 4:10 gears and 9.0:1 compression. It ran like a scalded cat - very low 12's, high 11's on sticky street tires. It can be a bit extreme on the street, and the mileage sucked. I don't agree that the comp 268 is the upper limit. I ran a lot more than that with no problems at all. The Comp Cams 280H is not out of reason with the right intake, gears, carb, and timing.