I've used a bunch of stock Mopar Hi Po 727 solid ( with no stamped opening spaces between the ears) flex plates both 6 bolt and 8 bolt,(non certified and SFI certified) on SB,BB and hemi motors with stock 11 converters all the way down to custom built 8 inch drag race converters with no issues so far
I know of other racers that had issues with the stock flex plates and they had the same issues with a after market SFI flex plate that had actual mechanical issues in their cars that caused the breakage to start with, the SFI stickers didn't fix it is my message
I have both types and part number Mopar Hi Po flex plates on my shelfs, the only noticeable differences between them is the decal and price