Not trying to throw a wrench in the works; when I began building my Magnum 5.9( now 408) I used my stock flex plate from my 75 Swinger w/318.
Been working fine for over 4 years. If I knew I could use the plate from the Magnum, I would have. I run a Modified 904 w/ 2,800 to 3,000 custom built torque converter.
I guess it depends how your 408 was balanced- if it was internally balanced you're fine with the 318 flexplate and a neutral convertor. My advice was for a stock 5.9.
