Not sure wheat ET level that NHRA mandates a SFI flexplate, as I haven`t raced an automatic transmission car in well over 20 years now, but back when I ran a C6 in my Fairmont, I used a factory 428 flexplate , and never had any issues with it. That was over 10 years of WOT transbrake launches at 5200 RPM, with low 1.4-high 1.3 60 foot times, running mid 10s in the 1/4 mile. And that flexplate was just a stock unit from one of my 428 cores, so it was likely over 30 years old at the time.