If it's about 7 1/2 inches long, it's a small block rod. B engines use 8.5?? rods, and r/b's use 9.328 rods. I don't have any 340 rods sitting around here, but I'm pretty sure you measure from tip to tip, the little flat spots on the balls. I know some of em have the little cups on em, but not sure how they measure them. I think they still measure from the two longest possible points. I don't think used rods would change length much as long as they aren't bent.