You must have bought it a while ago to say the least. They’ve been discontinued now for some time. I also have the proper B&M flex plate for the magnum swap. I think the old B&M part number is 10242. This is the one for the swap.
They haven’t been available for a while BUT Mancini says they have one. Not sure who the maker of it would be.
Mancini Racing - Balanced Flex Plate
They also list one for magnum to 904 trans