I have had a 400 small block with the short 5.56 rods for 40000 miles in a Cobra,with a solid flat tappet ,254 @0.50, 5.75 lift with 1.6 roller rockers .I ahve revved the sh#t out of it all the way to 7500rpm, often ,always, and it has never broken.The short rods are not ideal, and I have recently rebuilt with 3.750 steel crank and 6" h beam rods. The stock cast parts are very strong actually.