Get the flex plate with the engine. 3 out of 4 holes line up perfectly. The 4th hole will line up with the converter with a little bit of work with the rat tail file. I did this swap years ago. The B and M flex plate made to swap a magnum to an old converter, was not weighted right, always had a shake. I swapped to a stock 5.9 magnum plate, and the vibration was gone. The stock cam makes it feel like a good running 383, till the tach hits 4 grand, then the party is over.