I came across an interesting thread that mentioned something that should be looked into. Post 1 states this
"Apparently the counterbore at the back of the crank is too shallow causing the converter snout to push forward on the crank. Turns out I only had about .025 clearance, and I was told that the flexplate can flex .030 and maybe a bit more with a high HP car. Took .075 off of the converter snout and also shimmed back the converter with some washers and set pullout to .125 so I should have around .115 clearance now, hopefully I will be good this time."
Post 12 brings in the 4R70W.
http://forums.corral.net/forums/5-0...-crank-walk-issues-eagle-crank-aod-trans.html
The clearance he talks about is not between the torque converter and flexplate with the nuts/bolts loose. The clearance would be how much the flexplate can flex before the torque converter "nub" bottoms out in the crank and then starts to apply pressure to the back of the crank. (instead of just the felxplate flexing to absorb the pressure from the torque converter pushing forward)
I guess you could check the old torque converter / flexplate movement on the old 5.0 crank by having the flexplate bolted to the 5.0, and then the torque converter bolted to the flexplate and pushing at the back of the torque converter while measuring with a dial indicator.
And then do the same with the 347 and other torque converter (after pulling the engine to replace the thrust bearing)