When you get down into the 1.20s in 60, you would be surprised how much that stuff moves with motor plates. They work great for torsional forces. Are lousy for fore and aft movement when it has to control hundreds of pounds at 2+ Gs. The car is literally trying to drive out from under the heavy engine/trans assembly that doesn't want to move. Newton's first law of motion.

I always control converter clearance with shims/washers between the converter and flexplate, and bolt the flexplate directly to the crank. Some of the Powerglide bellhousing/adapter packages are done differently.