A Body Shorty Stainless Headers
A set of unequal length headers would be a massive improvement over stock manifolds.
Most of these cars aren't very fast anyways, so people aren't going to spend the extra money just for a small amount of horsepower, especially if the install is a *****. If you can sell them horsepower and a don't-take-the-whole-car-apart installation that works with Power steering and stick shift, you might have a mini gold mine on your hands.
I've gone low 10s in a Mustang with unequal length shorty headers.
Steve