As mentioned headers will help with the power/mileage providing you keep your foot out of it. However, I have noticed with my 3.0 since the headers, exhaust etc., I am getting the same mpg as before about (17 mpg combined) while enjoying the gain in power. I haven't been able to keep my foot off the throttle long enough to see how much mpg I have gained driving conservatively.

More than tread lightly. Leave it like you were never there, nor anyone else.
'90 X-cab 4.88's 33 BFG AT's, rr ARB, Headers, Ignition upgrade, cold air induction.
'91 X-cab 5.29's 315's BFG MT's, rr ARB, custom bumper and flatbed