Your converter will be fine with your 408. The 408 will have a lot of low end grunt with a cam that small. Why do you think you need a different flexplate?
Oh yeah, tell your brother that converter technology has made leaps and bounds in the last 20 yrs. or so. You can get a "tight" design 4000 stall converter that will drive ok on the street. Their nothing like the converters of the old days that slipped like a worn out clutch.