How's Candy Apple Red? Beautiful color. I know a guy with a 1983 F150 with that exterior color and red interior. His truck looks great. Candy Apple Red would be a good fit to a red interior and it matches what was available in 1991. The Ford paint code for that color is EU.

On edit: If you pick a red color rather than something completely different than what's on it now, it won't be so obvious that the truck was repainted. That way when you open the door or the hood, the original painted surfaces in the interior will somewhat match the new exterior color and won't clash.