fuel economy question

[kwin]

I am about to go purchase a 1991 2wd Suburban to replace the crew cab short box that I had built and lost in the big fire.
My plan is to lower the sub and convert to 5.3 ls power.
I am doing it for drivability and power but was curious as to what you guys have seen for changes in fuel economy from the factory 305/ 350 to the ls set up.

Thanks for your time.

Keith