Headers will definately help. They might not recoup their total value on one trip, but the engine needs to exhale. A properly designed system from headers back, will also have benefits.

I question the intake system. OEM? Have you added a higher flow air filter, or air intake? Stock design is restrictive to contain intake noise. Your TBI fuel system would benefit from a Throttle body Spacer. By design it is fuel injected, but the dual injectors spray fuel onto the throttle blades, which decreases atomization. Airaid, and Helix spacers repeatedly show the largest gains in their product lines, on TBI vehicles.
How is your ignition system? An aftermarket multiple spark ignition box (analog or digital) would be beneficial as well, offering a more complete burn.
I would check out Banks, they tend to have some packages for moblie homes, that increase efficiency (and power).