Which 340? 72 and 73 340's have 360 heads... and are you sure the 340 heads are better for the street than the "308" 360 heads?? You do need to install hardened seats in those 340 heads, which the 308's already have. And these heads are easily obtained, and bolt right on the 318. Infact, all those "hot" 340 parts bolt right to that 318. Heads, cam, intake, rods, crank, and that leaves us with ONE thing, the bore.... which.... equals... 22 cubic inches...