First off, we're dealing with a 350 cu. in. V-8 shoving a 1-plus ton hunk of steel. What kind of driving do you do (highway vs. city/local)? What kind of gas mileage are you getting? My '98 (w/305 and 4L60E tranny) gets 17-18mpg in a mix of highway and local driving (very little stop and go stuff). What kind of gas mileage are you expecting? What kind of final axle ratio are you running?
Z15 sorta hit the nail on the perverbial head: most of that stuff you can put on your ride to "improve" just about everything but your sex life, in a word, doesn't (okay that's really two words...just seeing if you're paying attention). There may actually *be* an improvement in your mileage or HP but you'd have to measure it with a micrometer, at redline and under laboratory condidtions; in the real world, not so much if any at all. The perverbial "Butt Dyno" is usually way out of calibration and bears no relation to the real world.
A TBI spacer typically has nothing to do with improving mileage; what it will do, supposedly, is slightly shift your torque peak higer or lower in the RPM range.
A cold air intake.....well I have a '98 and it came from the factory with a "cold air intake" as probably did yours; intake air is drawn from outside the engine compartment through the fender area. Under normal driving conditions the factory intake system, with a clean air filter, is more than adequate in flow requirements...only when you get into the upper RPM range will a low restriction intake show its stuff...providing you have a low restriction exhaust and a high flow throttle body to match.
Your dual exhaust.....you need to find out if the installer put your O2 sensor back...and if you have true duals, then that may have an effect on the sensed fuel mixture by the single O2 sensor. When you start screwing with engine management computer inputs you better know what you're doing. That little device probably has the greatest effect on your mileage since it tells the engine management computer how rich or lean the fuel mixture appears.....the correction, lean or rich, will be applied by the computer to get the ideal fuel mixture. Like all things computer, if you put garbage in you get garbage out; if the O2 sensor is giving erroneous inputs to the computer then your getting an erroneous fuel mixture.
Modern computer controlled engines are about as fuel efficient as they can be. You might be able to tweak some more MPG's but you'd probably need an engineering degree to do it. With the brick-on-wheels styling of the older style full-size GM trucks, aerodynamics (or the lack thereof) also has a lot to do with your mileage; wind resistance is the biggest sapper of gs mileage in these vehicles at higher speeds.
Keeping your engine in tune, running factory-sized tires and driving sensibly are probably the best things you can do to maintain the mileage you're getting. Putting in a higher (lower numerically) final drive might get you a little improvement in MPG numbers and installing a tonneau cover on the bed can help reduce wind drag and maybe help in mileage.
Mike
'98Sierra