I built a tranny for a friend who raced a 1970 340 Duster, it was a stock tranny to start with and I swapped the stock 2.65 gears and cluster to a set of stock 3.09 gears and cluster from a 1965 NP833 A body 4 speed, he had installed a Dana 60 with 5.57 gears in it with ladder bars and a slider set up on the stock leaf springs. I drove that car more than once at the track, you where really busy the first 300 feet
Let the clutch out at 7000 RPM and try and shift at 7000 RPM and wait for the 1/4 mile finish line to come up
He never broke that tranny or rear end