5.9 magnum, and 904 trans?????

Putting together a project, and need to know what flex plate to use. Also, what converter ?? 5.9 magnum,external balance, cast crank, 904 trans. This is for a street/ rat rod. 29" tire, 3:55 gears,2500#. It will be used for cruising.

http://www.magnumswap.com/automatic-transmission-details/

Thanks, looks like that's what I was looking for.