Want to put a magnum 5.9 in my 70 dart.

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
I had a conversation with Mancini Racing and they verified they are offering the Magnum balance offset flex plates. They are having a local company modify/add the offset notch. They did not have one on hand when I spoke to them so I have not seen one in person yet. If ordering one, make sure you are very specific that is is 5.9 Magnum balance you are seeking. This is a new offering to them and the salesmen on the phones may not be fully up to speed on this yet.