Crank end play, destroyed converter, 408 nightmare

if you push the converter rearward, and it is fully seated on both sets of splines and converter neck fully seated into front pump I believe there is a measurement from the bellhousing surface. It was in a factory service manual(possibly for a 70 Cuda), unfortunately I don't remember the distance. Maybe someone else can recall the measurement. I believe there is also a note in the FSM about an official tool you can employ to hold it in the proper position while installing the trans, which looked like something anybody could improvise without much effort.
For myself, I am just real careful when I install it, and am cognizant of making sure it doesn't slip forward during trans installation.