You don't want to do that on a stock transmission. You need, at minimum to install a valve body kit. Trans Go, Fairbanks, Superior, and B&M all make performance valve body kits that will set the transmissions up for proper performance shifting. TransGo and Fairbanks offer kits for different levels of use. Get the street/strip Level 2. I routinely ran a 727 Torqueflite, and now the A518(derived from the 727) to 6,000 rpm shift points with no breakage. I do have upgraded clutches, but the drums and other hardware are stock. It's not the rpm, it's the torque input that breaks parts. No way could a street Magnum spin high enough to sling parts. The guts for these transmissions go all the way back to the 440 Magnum, 426 Max Wedge, and 426 Hemi. Even though the stock clutch material is crap, the basic design is extremely robust. Anyway, the valve body kit will give you full manual control, while still allowing full auto when you put it in "D".