Stock 340 intakes

There should be a power gain with a better intake. Along with power and torque increase's, you should get a increase in mileage. It's part of the package. Tuned up well of course. It's now breathing better and leaner.

However, it may be a very very minimal increase.

Anytime you make the engine more efficent, it should show in power, torque and mileage. Tuning will eek out a tad more.
The vacuum advance is a must for mileage. Hook that baby back up and regain what you lost! You should get that 1 or 2 mpg's back easy.