360 Magnum flexplate / balancing experts !

Hi guys , my duster 360 magnum ( crate ) needs to have the trans rebuild so i was wondering since i have several trans laying around ( all for 318/360s LA ) what can i do if i want to put a reverse valve body 904 on my 360crate to finish the season ? Theres all kind of infos and theories but im kinda lost , i had this issue in 2008 and i had a b&m flexplate for my 727 and my cuda had a 5.9 magnum and a 4 speed trans , never played with 727 and 904 for a 360 magnum since then ! Any part number , infos ? Thanks guys !