Flexplate or converter? What stall?

Did the engine builder internally or externally balance it? You have to know that before proceeding.

Your 2800 stall converter will most likely stall 3500 behind a good torquey 408. The converter I had behind my old 360 stalls a bunch more behind my 408. That said you may not even need a converter.

Why did you put a super Victor intake on an engine that won't make usable power above 6000 rpm? That seems like a huge mis-match to me.