We've (my brother and I) have turned 318's at 6000 rpm's for years and them same motors ran like new when we sold 'em. One is still in ownership of a friend of mine, and that motor is now about 14 years since built and still going strong. I have an acquaintance that turns 7200 rpm's with a stock 318 (stock crank, rods, pistons) in stock eliminator in a dodge truck. He ran one motor 11 years before loosing a main. The bottom end of the 318 is 100 percent as strong as a 340. People put cams and headers on 340's and turn them 6000 rpm's all the time.