1990 280 sport with stock 454 mags and EMI thunder manifolds with the cast risers. The internal flaps are burned out but the boat has external flappers. Do you think I need internal also?
Thanks

No sir. As long as you have the external flappers you should be fine. I have never used both at the same time and I have only ever had problems when I tried running with none at all in my old baja. And that was when we stopped for a potty break for the wife and I took a tug boat wake in the stern.

I wouldn't even think about replacing the internal flaps if you have the external.