I don't beleive the intake will change the MPG much at all.

What will make a big differance is getting the fuel atomized better. When you pull the intake you will see where the fuel stream shoots right on the PR pinch then onto the floor, well that fuel never gets atomized properly. Put some accell injectors in it, they shoot out a fine spray instead of a straight stream. Another thing that will help is get some quench in it, cheap way is to deck the block about .060 (measure your block don't take my word)and run a .039 head gasket, that should get you about .040 quench and it will bump the compression a little, that should bring the MPG up significantly as well as the low RPM TQ and then you can bump up the gear ratio and really help out the MPG. Another thing is you can get the computer leaned out a little by one of the programmer companys.

Shrinking the plenum will make it more throttle responsive but thats about it. The computer will make adjustments for any A/F ratio changes. The mods I mentioned will make more use of the fuel squirted into the engine so you will make the same power with lower throttle angle and less fuel squirted in, the computer will actually lean out the mix more because the O2 in the exhaust is consumed more completly and it will think it is rich when there is less O2.