We`ve tached stock 350`s to 6000 many a time and held it there over 15 seconds without any issues. Cool thing about small blocks tached that high is they sound really cool. In my tech college days my instructor told us about how he raced in the late 60`s. He had a tubbed 55 chevy, 283 bored to 4 inches, 3.00 inch stroke crank, a massive solid lifter cam, fully ported camel back heads, big valves, monster valve train, tunnel ram dual quad with twin 585 holley carbs, aftermarket rods and forged dome pistons. The rear had 5.13 gears and the tranny was a 3 speed GM. He`d pull up, do his burn out, then pull to the line, he`d push in the clutch and tach it to 9500 RPM, when the light went green he`d side step the clutch. His words on it were "If it didn`t break, you went fast" He said he never lost when it didn`t break, but it broke often. He showed me parts he still had. Twisted axles, grenaded tranmissions, exploded clutches, gutted differentials, splintered bell housings.
He said what he hated the most was when the clutch exploded it took off part of his toe and left a scar on his leg from a deep scratch. He said another time he was at the line tached to 9500 and the harmonic balancer came off. He said those engines had press fit only balancers, but there after he had the crank drilled to make sure it didn`t come off again. He said when the balancer hit the track at that speed it sounded like a bomb went off and pieces flew everywhere. Needless to say the track manager nicely asked him to leave lol.