When do I use an S.F.I. Flex plate

By rule book a SFI flexplate is required went you run faster then 10.99.......

Depending on your torque converter as to whether it used 5/16 or 7/16 thread bolt...your stock one would need to be redrilled to fit the larger thread.....not much meat there...

I just bought the MP SFI flexplate for the 408 going in 71 Duster....I have one of those shown in the picture but did not like how much it weighted...